Add preprocess docs, fix garbage in/out bug

Add documentation on how to use the preprocess option.

Fix a small bug when a user returns back a non-string value.
Never mind: 
- http://www.adequatelygood.com/Object-to-Primitive-Conversions-in-JavaScript.html
- http://stackoverflow.com/questions/3945202/whats-the-difference-between-stringvalue-vs-value-tostring