Include option to keep emacs-included ctags, remove by default.

This forrnula conflicts with exuberant ctags, a more featureful ctags package in homebrew.

The main emacs formula has chosen to remove ctags by default and make it an optional binary. I would like this option copied into this formula.

If you'd prefer that ctags be installed by default and the option be to not include it instead, I can rework the changes.

Thank you!