Make work in a browser (remove fs and vm dependencies)

Would it be possible to make js-module-formats work in the browser?  It would have to remove the `fs` and `vm` dependency.  It doesn't seem like `fs` is needed or used, and I'm being lazy right now and not tracking down why `vm` is needed.

I'd like to make https://github.com/bitovi/transpile able to load in the browser with SystemJS. Let me know what you think.  Thanks!
