[Feature] Shell completion

[alydevs]

Use case: I was hoping to be able to autofill pho view str<tab> with installed packages starting with that string.

I'm unfamiliar with Go, but I plan to investigate making a PR implementing https://github.com/posener/complete maybe

[zyrouge]

Seems like urfave/cli has support for autocompletion. I'll probably take some time to look into this, might as well give a better working implementation.