Load css files via browserify

via: https://github.com/css-modules/css-modulesify/issues/29#issuecomment-132832636

> A thing I've noticed but haven't had time to investigate: css-modulesify outputs no css on subsequent builds with https://github.com/bjoerge/rebundler. I suspect this is because css files aren't being added to the browserify cache because they're required by a custom loader.
