Add support for require calls

Resolves #147

Todo:

- [ ] ~~Implement [client-side caching](https://github.com/shama/browser-module-cache)~~ This will be added in a future update
I'm undecided about this. Im going to rename it codeExtractor, which makes its purpose more clear. Separate from that is the moduleBundler, which interprets the JavaScript. Do you think it makes sense for the codeExtractor to interface w. the bundler?