Pytokei

Added a small CLI to run the program from the console.

Fixed:
- `PyLanguages.report_compact_plain` now counts correctly the
summarised languages.


Added typing info for:
- `Languages.total_plain`.
- `Languages.report_compact_plain`.

Updates/Fixes in docs:

- `index.md`: add binary versions available.

- Time execution comparison example in docs.

- Updated `mkdocs.yml`, changed Examples to Usage.

- Add API reference in docs.

First version released to PyPI.