Support for Tidy

Replacement for [Pull Request 648](https://github.com/josegonzalez/homebrew-php/pull/648). I tried squashing the last one, but couldn't purge the old commits out on github even though it worked locally, so I'm closing the old in favor of the new.

The tidy formula just recompiles using the --with-tidy flag, but it's a nice shortcut for those who are not advanced brew users.