Vendor buildifier

[TheButlah]

It would be awesome if the buildifier hook could download buildifier instead of assuming that it is system installed. Buildifier is a go program, so it can simply be wgetted from the github repository with a simple check to determine whether the os is windows, mac, or linux.

[jlebar]

Agree that would be way better.

I'd be happy to take a patch, now that I'm subscribed to updates to this repo. :)