Provide a top-level transformation hook

This adds in a simple hook to be able to get the transformed source without it being immediately written to disk. I did this as I wanted to use streamline with `gulp`, in a plugin [here](https://github.com/calvinwiebe/gulp-streamlinejs). I realize that most of the individual __transform__ functions for the different runtimes are exposed. However, there is a lot of useful option parsing and also plain `.coffee` compiling provided in `compile._js`. 

Details:
1. Added a `noWrite` option.
2. Factored out some common code to be used by both `exports.compile` and `exports.transform`
3. I ran `build.sh` to generate a new `./lib/callbacks/compile.js`. I hope that was appropriate.