Add support for bash completion

[fredemmott]

No description provided.

[lexidor]

I am looking at \readline_completion_function() right now. This requires the cli request to already be running. This is not how hh-clilib currently works. (The Hack code runs when the command is complete instead of while you've typing it.)

If not that, you've left using something like complete. This wouldn't work on the first run, since we've had no chance to "mess" with the user's shell yet. I don't know if this is something we want to do either...

Did you have anything in mind half a decade ago that I am missing? It's okay if you don't remember. (I sure wouldn't. 🙂)

We do get a chance to do so before the first run with a composer plugin that hooks into the composer install's post install hook, but that feels even more sketchy. Just imagine composer install 's output s/hhvm-autoload/hh-clilib/g.