Add Background parsing

[mathpunk]

I'm finding your parser very useful, thank you. There is a thing I'll want to add, and that is the ability to read Background sections. If you have advice -- I've written a modest amount of Clojure, but I've read very little -- let me know and I will try and submit code.

[mathpunk]

Do you think I can just find everything that's a scenario and like, duplicate it but for a Background keyword?