Allows injections into the null grammar

Closes #9 

This makes the following work inside untitled editors in Atom:

![screen shot 2014-03-06 at 11 17 22 am](https://f.cloud.github.com/assets/671378/2349454/1c5149c8-a564-11e3-9d0b-2c7a097f1368.png)