More core commands

Move more commands (and stuff) out of `Contributions`. Also, make some commands that only apply when `HOMEBREW_DEVELOPER` is set. This bit is maybe a bit more controversial.

CC @adamv @mistydemeo @jacknagel @tdsmith @asparagui for thoughts and review.
May be a good time to get rid of some of the options in this command? Does anyone use `--link`, or `--server`?