Fish completion support

[Ellmen]

Re: #56

This adds support for completions for the fish shell. It adds an optional argument to the --completions flag to specify a shell to create completions for.

Usage looks like:
widget -- --completion or widget -- --completion bash
for bash completions or
widget -- --completion fish
for fish completions

[dbieber]

Thanks for this contribution!
Props for your thoroughness with docs and tests :)

[dbieber]

Looking forward, we'd eventually like to replace our use of argparse in parser.py with fire.

When we do so, "completion" will become an argument of a function with default value 'None'. At that point '--completion' will be treated as True, '--completion bash' will be 'bash', and '--completion fish' will be 'fish'. So, like in your code, we'll treat True as 'bash'.

[Ellmen]

Happy to help! Sounds good - it should be relatively easy to add zsh support the same way

[dbieber]

That would be great -- #64 requests it.
I also welcome you to read the doc in #32 (here) and let me know if you have thoughts on this or want to work on it. It's a proposal to eliminate individual per-CLI completion scripts in favor of a single completion script for all Fire CLIs.