Use begin/end patterns for single line comments

This allows injection grammars to parse what is in between such as the language-todo grammar.

### Before
![screen shot 2014-11-17 at 5 24 22 pm](https://cloud.githubusercontent.com/assets/671378/5081076/cd2cb992-6e7e-11e4-8006-d6db444ad412.png)

### After
![screen shot 2014-11-17 at 5 24 36 pm](https://cloud.githubusercontent.com/assets/671378/5081072/bc85310a-6e7e-11e4-9c04-bdf4a446bb89.png)

Closes https://github.com/atom/language-todo/issues/13