Nicer frontend exception display (streamlit#490)

- Defines a new exception class, `StreamlitAPIException`, that's used for exceptions resulting from improper usage of the streamlit API. (That is, `StreamlitAPIException` is meant for "user errors", like calling a DeltaGenerator function with bad inputs.)
- `Exception.proto` has a new field, `message_is_markdown`, which is True when the exception being sent is a `StreamlitAPIException` instance. Markdown formatting in the exception message will be nicely rendered on the frontend.
- `StreamlitAPIException` stack traces get all Streamlit entries pruned before being marshalled into the exception proto. This means that when a user misuses a Streamlit API, they no longer see a bunch of ScriptRunner and DeltaGenerator lines in the stack trace we show them.
- I updated most (all?) user-facing errors to use StreamlitAPIException, for the nicer formatting.
- Non-API-related errors, originating from the bowels of Streamlit, will _not_ be specially pruned/formatted, so this shouldn't hinder us when debugging Streamlit itself.

Fixes #438

* Create CONTRIBUTING.md

* Adding missing key parameter to text_input (#571)

* Adding key parameter to number_input

* Test

* linter

* Fix trailing whitespace (#562)

* Explicitly use bash in Makefile (#556)

The Makefile uses bash-isms like `[[ $(PY_VERSION) == "3.6.0" || $(PY_VERSION) > "3.6.0" ]]`
that don't work in POSIX sh.  So let's be explicit.

This fixes a problem with the build in Ubuntu, where you currently get `[[: not found`.

* Extends capability to send config keys by param to "streamlit hello" command (#527)

* add parameter config options to "streamlit hello"

* added param config keys to "config show" command

* fix lint

* add custom env var names for streamlit cmd options

* cleanup commit

* Fix cypress tests (#559)

* update disconnected snapshot

* update empty_datafrasmes snapshots

* fix pyplot_kwargs cypress test

* fix Cypress tests: return all promises based image comparisons so IO does not conflict

* fix typo on vega_lite_chart.spec.ts

* remove ribbon decoration from the top during e2e tests

* restore load check to pyplot cypress test

* remove ribbon from add_rows and pyplot

* Nicer frontend exception display (#490)

- Defines a new exception class, `StreamlitAPIException`, that's used for exceptions resulting from improper usage of the streamlit API. (That is, `StreamlitAPIException` is meant for "user errors", like calling a DeltaGenerator function with bad inputs.)
- `Exception.proto` has a new field, `message_is_markdown`, which is True when the exception being sent is a `StreamlitAPIException` instance. Markdown formatting in the exception message will be nicely rendered on the frontend.
- `StreamlitAPIException` stack traces get all Streamlit entries pruned before being marshalled into the exception proto. This means that when a user misuses a Streamlit API, they no longer see a bunch of ScriptRunner and DeltaGenerator lines in the stack trace we show them.
- I updated most (all?) user-facing errors to use StreamlitAPIException, for the nicer formatting.
- Non-API-related errors, originating from the bowels of Streamlit, will _not_ be specially pruned/formatted, so this shouldn't hinder us when debugging Streamlit itself.

Fixes #438

* #558 fix bart_vs_bikes example (#599)

* Error is an undefined name in run_on_save.py (#605)

python3 -c "Exception"  # No problems
python3 -c "NotImplementedError"  # No problems
python3 -c "Error"  # NameError: name 'Error' is not defined

[flake8](http://flake8.pycqa.org) testing of https://github.com/streamlit/streamlit on Python 3.8.0

$ __flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics__
```
./lib/tests/streamlit/scriptrunner/test_data/compile_error.py:22:9: E999 SyntaxError: invalid syntax
because i am a compile error!
        ^
./lib/streamlit/hashing.py:118:32: F821 undefined name 'string_types'
            or isinstance(obj, string_types)
                               ^
./lib/streamlit/hashing.py:255:34: F821 undefined name 'string_types'
            elif isinstance(obj, string_types):
                                 ^
./lib/streamlit/hashing.py:306:42: F821 undefined name 'string_types'
                or (isinstance(obj.name, string_types) and os.path.exists(obj.name))
                                         ^
./lib/streamlit/hashing.py:394:34: F821 undefined name 'string_types'
            if not isinstance(n, string_types) or not n.endswith(".<lambda>")
                                 ^
./lib/streamlit/__init__.py:547:14: F821 undefined name 'source_util'
        with source_util.open_python_file(filename) as source_file:
             ^
./lib/streamlit/caching.py:81:18: F821 undefined name 'Dict'
_mem_cache = {}  # type: Dict[string, CacheEntry]
                 ^
./lib/streamlit/caching.py:81:18: F821 undefined name 'string'
_mem_cache = {}  # type: Dict[string, CacheEntry]
                 ^
./lib/streamlit/hello/demos.py:213:9: F821 undefined name 'reload'
        reload(sys)
        ^
./lib/streamlit/elements/vega_lite.py:41:22: F821 undefined name 'dict_types'
    if type(data) in dict_types and spec is None:
                     ^
./lib/streamlit/elements/vega_lite.py:89:31: F821 undefined name 'dict_types'
        if type(data_spec) in dict_types:
                              ^
./lib/streamlit/elements/media_proto.py:76:22: F821 undefined name 'string_types'
    if type(data) in string_types:
                     ^
./lib/streamlit/elements/media_proto.py:122:25: F821 undefined name 'string_types'
    if isinstance(data, string_types) and url(data):
                        ^
./lib/streamlit/elements/media_proto.py:154:25: F821 undefined name 'string_types'
    if isinstance(data, string_types) and url(data):
                        ^
./lib/streamlit/elements/lib/dicttools.py:125:30: F821 undefined name 'native_dict'
        if type(v) in (dict, native_dict):
                             ^
./lib/streamlit/elements/lib/dicttools.py:129:42: F821 undefined name 'native_dict'
                if type(child) in (dict, native_dict):
                                         ^
./docs/conf.py:206:41: F821 undefined name 'github_doc_root'
            "url_resolver": lambda url: github_doc_root + url,
                                        ^
./docs/api-examples-source/widget.time_input.py:3:39: F821 undefined name 'datetime'
t = st.time_input("Set an alarm for", datetime.time(8, 45))
                                      ^
./examples/run_on_save.py:68:11: F821 undefined name 'Error'
    raise Error("Windows not supported")
          ^
./examples/run_on_save.py:71:11: F821 undefined name 'Error'
    raise Error("Unknown platform")
          ^
1     E999 SyntaxError: invalid syntax
19    F821 undefined name 'datetime'
20
```
__E901,E999,F821,F822,F823__ are the "_showstopper_" [flake8](http://flake8.pycqa.org) issues that can halt the runtime with a SyntaxError, NameError, etc. These 5 are different from most other flake8 issues which are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

* Undefined name: import datetime in widget.time_input.py (#606)

[flake8](http://flake8.pycqa.org) testing of https://github.com/streamlit/streamlit on Python 3.8.0

$ __flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics__
```
./lib/tests/streamlit/scriptrunner/test_data/compile_error.py:22:9: E999 SyntaxError: invalid syntax
because i am a compile error!
        ^
./lib/streamlit/hashing.py:118:32: F821 undefined name 'string_types'
            or isinstance(obj, string_types)
                               ^
./lib/streamlit/hashing.py:255:34: F821 undefined name 'string_types'
            elif isinstance(obj, string_types):
                                 ^
./lib/streamlit/hashing.py:306:42: F821 undefined name 'string_types'
                or (isinstance(obj.name, string_types) and os.path.exists(obj.name))
                                         ^
./lib/streamlit/hashing.py:394:34: F821 undefined name 'string_types'
            if not isinstance(n, string_types) or not n.endswith(".<lambda>")
                                 ^
./lib/streamlit/__init__.py:547:14: F821 undefined name 'source_util'
        with source_util.open_python_file(filename) as source_file:
             ^
./lib/streamlit/caching.py:81:18: F821 undefined name 'Dict'
_mem_cache = {}  # type: Dict[string, CacheEntry]
                 ^
./lib/streamlit/caching.py:81:18: F821 undefined name 'string'
_mem_cache = {}  # type: Dict[string, CacheEntry]
                 ^
./lib/streamlit/hello/demos.py:213:9: F821 undefined name 'reload'
        reload(sys)
        ^
./lib/streamlit/elements/vega_lite.py:41:22: F821 undefined name 'dict_types'
    if type(data) in dict_types and spec is None:
                     ^
./lib/streamlit/elements/vega_lite.py:89:31: F821 undefined name 'dict_types'
        if type(data_spec) in dict_types:
                              ^
./lib/streamlit/elements/media_proto.py:76:22: F821 undefined name 'string_types'
    if type(data) in string_types:
                     ^
./lib/streamlit/elements/media_proto.py:122:25: F821 undefined name 'string_types'
    if isinstance(data, string_types) and url(data):
                        ^
./lib/streamlit/elements/media_proto.py:154:25: F821 undefined name 'string_types'
    if isinstance(data, string_types) and url(data):
                        ^
./lib/streamlit/elements/lib/dicttools.py:125:30: F821 undefined name 'native_dict'
        if type(v) in (dict, native_dict):
                             ^
./lib/streamlit/elements/lib/dicttools.py:129:42: F821 undefined name 'native_dict'
                if type(child) in (dict, native_dict):
                                         ^
./docs/conf.py:206:41: F821 undefined name 'github_doc_root'
            "url_resolver": lambda url: github_doc_root + url,
                                        ^
./docs/api-examples-source/widget.time_input.py:3:39: F821 undefined name 'datetime'
t = st.time_input("Set an alarm for", datetime.time(8, 45))
                                      ^
./examples/run_on_save.py:68:11: F821 undefined name 'Error'
    raise Error("Windows not supported")
          ^
./examples/run_on_save.py:71:11: F821 undefined name 'Error'
    raise Error("Unknown platform")
          ^
1     E999 SyntaxError: invalid syntax
19    F821 undefined name 'datetime'
20
```
__E901,E999,F821,F822,F823__ are the "_showstopper_" [flake8](http://flake8.pycqa.org) issues that can halt the runtime with a SyntaxError, NameError, etc. These 5 are different from most other flake8 issues which are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

* Define github_doc_root in docs/conf.py (#608)

Fixes: #607

* Fix matplotlib backend on all platforms (#595)

* PlotlyChart: Force recreation of <Plot> in render (#629)

Plotly charts are not always re-rendering properly. 

This seems to be a react-plotly bug; I made a standalone repro case and submitted it to them here plotly/react-plotly.js#167.

Fixes #512

* Update bug_report.md

* Update feature_request.md

* Replace fetch with axios in JS websocket code (#598)

* Using Axios

* Checking timeout

* lock file

* Cleaning stale elements when the reportHash changes (#617)

* Cleaning stale elements when the reportHash changes

* Cleaning

* Fixing Metrics

* Removing reportName and some cleaning

* fix for #618: pandas and numpy import unification in examples (#633)

* Fixing print as pdf (#566)

* Fixing print as pdf

* Fixing firefox print

* Fixing double font for firefox

* Support GitHub blobs and gists by transforming normal to "raw" URLs. (#645)

* starting conditions from @aritropaul's PR

* added test scaffolding

* snakecase and mild refactor of function

* refactor into util.py

* added fixture structure

* Update bug_report.md

* Update feature_request.md

* rebasing cli.py to streamlit/develop to fix missing commits

* linting

* fleshed out the tests. should collect more fixtures.

* working regular expression and tests. needs confirmation.

* copied in yarn.lock from streamlit/streamlit/develop

* make sure not to spuriously replace "blob", eg. username "theblob"

* mild function reorg and comment style change

* Undefined names: missing imports (#632)

* Undefined names: missing imports

* noqa: F821 because linters don't like magic

* Only install watchdog on Mac if gcc is working (#587)

* check if gcc is working before making watchdog a dependency

* black

* Pull these conditions into their own boolean constants, for readability:

* only check for xcode if we're on darwin

* Adding sidebar to main concepts (#507)

* Adding sidebar to main concepts.

* Draft of sidebar content.

* Addressing Thiago's comments.

* report -> app.

* Addressing Thiago's comment in PR507.

* Fix empty_charts example

* Improve docs about LaTeX

* Update examples in DeltaGenerator docs and api.md

* Fix embed=true padding and scrollbars

* Fix wording in "App snapshot" dialog

* Version 0.50.0

* Improve how exception tracebacks render

* Version 0.50.1

* Changelog for 0.50.1

* Make CSS changes to stack traces only apply to stack traces -- not any pre/code in an stException

* Version 0.50.2

* Fix date in changelog :D

* Update notices.