Distinguish the highlighting of macros from atoms

A super-small tweak, trying to get macros highlighted different from atoms. Emacs Erlang highlighting was used as a reference (the attached image is an example of how a sample snippet would look in Emacs with Solarized color scheme - you can see that the same highlighting is used for atoms as for strings). Of course Emacs highlighting is not dogmatic so if you think it is better as it is, feel free to skip this pull request. Though in my opinion it helps to distinguish between atoms and macros as Emacs does, since they can look really similar and one can miss the '?' sign treating a macro as an atom (if it's in lowercase).


![Screen Shot 2013-01-11 at 11 31 12 PM](https://f.cloud.github.com/assets/1038534/61291/ab261d10-5c3e-11e2-934b-1b8746d73a51.png)

And keep up the really good work with the project!
Best Regards,
Sergey