Improve modularity

It's great simple module, just two things that bothers me, is AMD boilerplate and no modularity (parse and stringify are ported in single module).

Do you consider updating it, so:
1. It's pure & clean CJS module, (which still can easily be bundled to AMD via external tool, and served in various (minified etc) forms but outside of repository).
2. It's split into three modules: `parse`, `stringify` and `index` (main entry that serves both), this one would help reduce dead code on browser-side if we're after just one functionality

I can make pull request, but I'm not sure whether you consider such update.