Add completion for popular shells

[linozen]

It would be really nice to have completion for all the popular shells such as:

• [ ] bash
• [ ] zsh
• [ ] fish

[nicorikken]

Ideally instrument it in a way that parameters in Python and completions can easily be kept in sync to prevent drift. One option is to generate them from the argparse definitions. Another approach can be to generate them from the helpfile or manpage as some external tools do. For Zsh at least it is worth considering a mechanism of installing completions.

[rpatterson]

Perhaps argcomplete?