Add completion for targets

[alerque]

CaSILE rules have proliferated so I now have some projects with thousands of possible outputs.

We need some way other than knowing and typing to make these discoverable. I would like some sort of completion routine, maybe with a fuzzy-match selector.

We probably need to cache the list of potential outputs somehow because it takes make a long time to put everything together.

c.f. https://github.com/kyu08/fzf-make

[alerque]

This is also pending Clap having dynamic completion support. It is not available in an unstable state, but not quite ready to use.