#8 shell (and open) command tab completion

No idea how to do this but it would be awesome.

OpenBSD's mg doesn't support this nor do I have a huge use for this so I don't have a huge reason to do this but it would be cool to do once ait gets to a stable and finished* state.

• ait will likely never be "finished" but I'd like to have it at a point where there aren't obvious bugs and that it works for mostly everything.