Shtab

* fix `argparse` dependency on py<3.2 (5 -> 6)

- zsh: fix subcommands containing hyphens `-` (4)
- documentation updates
+ updated usage instructions
+ added contributing guide
- added tests
+ tested main
+ added coverage reports
- minor code tidy

First official `shtab` release! :confetti_ball:

Check out https://github.com/iterative/shtab for documentation.

-- *much tab such complete* :100: :tada: