Allow injection grammars in comments

Using a begin/end pattern for single line comments allows injectiongrammars such as TODO and hyperlinks to highlight in comments.

### Before

![screen shot 2013-08-31 at 10 40 18 am](https://f.cloud.github.com/assets/671378/1062689/268b0696-1265-11e3-9041-c9632ffe2545.png)

### After

![screen shot 2013-08-31 at 10 43 26 am](https://f.cloud.github.com/assets/671378/1062690/30461662-1265-11e3-97eb-2b82cd2be1bb.png)