Add support for CDPATH for `.cd`

pkgsrc change: add support for pkg_alternatives

### HEAD

#### Features

* Add Pry::Testable, an improved modular replacement for PryTestHelpers.
  **breaking change**.

See pull request [#1679](pry/pry#1679).

* Add a new category module: "Pry::Platform". Loosely related to #1668 below.

See pull request [#1670](pry/pry#1670)

* Add `mac_osx?` and `linux?` utility functions to Pry::Helpers::BaseHelpers.

See pull request [#1668](pry/pry#1668).

* Add utility functions for drawing colorised text on a colorised background.

See pull request [#1673](pry/pry#1673).

#### Bug fixes

* Fix a case of infinite recursion in `Pry::Method::WeirdMethodLocator#find_method_in_superclass`
  that users of the [Hanami](http://hanamirb.org/) web framework experienced and
  reported since 2015.

See pull request [#1639](pry/pry#1689).

* Fix a bug where Method objects were not returned for setters inherited
  from a default (Pry::Config::Default). Eg, this is no longer an error:

      pry(main)> d = Pry::Config.from_hash({}, Pry::Config::Default.new)
      pry(main)> d.method(:exception_whitelist=) # Error

See pull request [#1688](pry/pry#1688).

* Do not capture unused Proc objects in Text helper methods `no_color` and `no_paging`,
  for performance reasons. Improve the documentation of both methods.

See pull request [#1691](pry/pry#1691).

* Fix `String#pp` output color.

See pull request [#1674](pry/pry#1674).

### 0.11.0

* Add alias 'whereami[?!]+' for 'whereami' command. ([#1597](pry/pry#1597))
* Improve Ruby 2.4 support ([#1611](pry/pry#1611)):
  * Deprecated constants are hidden from `ls` output by default, use the `-d` switch to see them.
  * Fix warnings that originate in Pry while using the repl.
* Improve completion speed in large applications. ([#1588](pry/pry#1588))
* Pry::ColorPrinter.pp: add `newline` argument and pass it on to PP. ([#1603](pry/pry#1603))
* Use `less` or system pager pager on MS Windows if it is available. ([#1512](pry/pry#1512))
* Add `Pry.configure` as an alternative to the current way of changing configuration options in `.pryrc` files. ([#1502](pry/pry#1502))
* Add `Pry::Config::Behavior#eager_load!` to add a possible workaround for issues like ([#1501](pry/pry#1501))
* Remove Slop as a runtime dependency by vendoring v3.4 as Pry::Slop.
  People can depend on Slop v4 and Pry at the same time without running into version conflicts. ([#1497](pry/pry#1497))
* Fix auto-indentation of code that uses a single-line rescue ([#1450](pry/pry#1450))
* Remove "Pry::Config#refresh", please use "Pry::Config#clear" instead.
* Defining a method called "ls" no longer breaks the "ls" command ([#1407](pry/pry#1407))
* Don't raise when directory permissions don't allow file expansion ([#1432](pry/pry#1432))
* Syntax highlight <tt> tags in documentation output.
* Add support for BasicObject subclasses who implement their own #inspect (#1341)
* Fix 'include RSpec::Matchers' at the top-level (#1277)
* Add 'gem-readme' command, prints the README file bundled with a rubygem
* Add 'gem-search' command, searches for a gem with the rubygems.org HTTP API
* Fixed bug in the `cat` command where it was impossible to use line numbers with files ([#1349](pry/pry#1349))
* Fixed uncaught Errno::EOPNOTSUPP exception when $stdout is a socket ([#1352](pry/pry#1352))
* Display a warning when you cd'ed inside a C object and executed 'show-source' without arguments ([#691](pry/pry#691))
* Make the stagger_output method more reliable by reusing possibly available Pry instance ([#1364](pry/pry#1364))
* Make the 'gem-install' message less confusing by removing backticks ([#1350](pry/pry#1350))
* Fixed error when Pry was trying to load incompatible versions of plugins ([#1312](pry/pry#1312))
* Fixed bug when `hist --clear` led to ArgumentError ([#1340](pry/pry#1340))
* Fixed the "uninitialized constant Pry::ObjectPath::StringScanner" exception during autocomplete ([#1330](pry/pry#1330))
* Secured usage of colours with special characters (RL_PROMPT_START_IGNORE and RL_PROMPT_END_IGNORE) in Pry::Helpers::Text ([#493](pry/pry#493 (comment)))
* Fixed regression with `pry -e` when it messes the terminal ([#1387](pry/pry#1387))
* Fixed regression with space prefixes of expressions ([#1369](pry/pry#1369))
* Introduced the new way to define hooks for commands (with `Pry.hooks.add_hook("{before,after}_commandName")`). The old way is deprecated, but still supported (with `Pry.commands.{before,after}_command`) ([#651](pry/pry#651))
* Removed old API's using `Pry::Hooks.from_hash` altogether
* Removed hints on Foreman support (see [this](ddollar/foreman#536))
* Fixed support for the tee command ([#1334](pry/pry#1334))
* Implemented support for CDPATH for ShellCommand ([#1433](pry/pry#1433), [#1434](pry/pry#1434))
* `Pry::CLI.parse_options` does not start Pry anymore ([#1393](pry/pry#1393))
* The gem uses CPU-less platforms for Windows now ([#1410](pry/pry#1410))
* Add `Pry::Config::Memoization` to make it easier to implement your own `Pry::Config::Default` class.([#1503](pry/pry#1503))
* Lazy load the config defaults for `Pry.config.history` and `Pry.config.gist`.