brew search <tapname>

I converted two colleagues to homebrew. Yay. And two times, I was asked which formulae were added by a certain tap. (Of course `ls $(brew --prefix)/Library/Taps/samueljohn-python` works).

I propose `brew search <tapname>` to show the contents of a tapped directory. 

Further, after `brew tap <tapname>`, I would appreciate that the tapped formulae would be listed or at least the proposed `brew list <tapname>` should be mentioned. This is not yet implemented by this PR.