bash completion for hab

[srenatus]

For discovering how to use habitat, it would be super-convenient to

1. have basic completion: hab p[TAB] -> hab pkg etc.,
2. have more advanced completion:
   • hab pkg exec [TAB] -> hab pkg core/hab-pkg-`
   • hab pkg exec core/hab-pkg-do[TAB] -> hab pkg exec core/hab-pkg-docker,
   • hab pkg exec core/hab-pkg-docker [TAB] -> ``hab pkg exec core/hab-pkg-docker hab-pkg-dockerize`

clap-rs/clap#376 is probably relevant for (1.); (2.) is probably harder.

[bookshelfdave]

first cut here: #698, nothing fancy at this point though.

[fnichol]

Is #698 sufficient for the time being to close this out?

[juliandunn]

I'm going to close this as sufficiently done - contributors can improve on it, but we have something.

[srenatus]

@metadave Thaaaanks! 🎉