[RFC] Add a dynaconf show command to view a single setting

**Is your feature request related to a problem? Please describe.**
It would be nice to have a `dynconf show` command to display information for a single setting.

**Describe the solution you'd like**
```
$ dynaconf show MEDIA_ROOT
Name: MEDIA_ROOT
Type: str
Value: /var/lib/dynaconf/media
Location: /etc/dynaconf/settings.toml
...
```