Ignores node modules.

This is an example PR that resolves #4.

The problem was that I was compiling all of the node modules. This particular project has lots of node module dependencies, and it was *seriously* slowing things down.

With this fix, my build time went from 30secs to 3secs.

Props to @thejameskyle for sharing [this comment](https://github.com/6to5/6to5/issues/144#issuecomment-62532353) with me.