Add callback to precompile function, make sure console.log only happens when verbose

Two things added here. The first is a callback for the exported precompile function, which is really useful if you are integrating this tool programmatically. The second is that there was no way to prevent `console.log`s from being fired for some of them, this is not something you want when integrating with another library. I set all the `console.log` calls only to fire when the verbose option was specified.

Nothing crazy, just a little patching I needed to be able to integrate this with [roots](http://roots.cx) for an upcoming client-side js framework update. Really awesome work on this library, this saved me so much time :smile: