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.
I got that from understanding what CoffeeScript outputs. I did some research. Seems there isn't any difference between `'' +`, `toString()`, or `String()` as far as functionality (I think?) Anyway I can't find any good documentation or bogs talking about the subject of pros/cons of each and about the only thing I found was [this jsPerf](http://jsperf.com/string-coercion-vs-tostring/2).

Does anyone have any insight?