Add rabl to list of supported filetypes

This is a minor one but a super handy one to have, since `*.rabl` files are all 100% Ruby anyway.

![screenshot 2014-04-02 08 40 43](https://cloud.githubusercontent.com/assets/58783/2591538/7cad746e-ba6c-11e3-9822-b291f8e8fc9a.png)

Let me know if anything else needs to be added to this PR; I'm happy to oblige!