Add Bash completion

[aude]

I use pacmatic every day, and am very happy to have it's automatic .pac{new,save} and mailing list checking as I'm upgrading.

One thing I've been missing is Bash completion for pacmatic. So I set out to create a Bash completion that's just a shim to pacman's completion.

This pull request adds that, wish some elaborate comments explaining how it's done (I don't think it's obvious without comments, because the bash-completion system has little documentation).

Would be very happy to see this code merged, as I would use the code every day :smile: In any case, thanks a lot for pacmatic!