Add syntax highlighting for YARD documentation comments

![screen shot 2014-08-31 at 10 21 55 am](https://cloud.githubusercontent.com/assets/1038121/4102595/8b9a80c8-3133-11e4-818f-9d3255716867.png)

A number of the Ruby projects I work on use [YARD](http://www.yardoc.org) for documentation comments. I've found syntax highlighting of the comments to be useful and wanted to integrate that into Atom. The YARD support is scoped to single-line comments only, so it shouldn't affect people that don't use YARD.