Add support for transpiling ES6 Unicode regular expressions

**Do not merge this yet!**

@arv This is an initial draft of how I think this could work. The build fails for obvious reasons (see below) but do you have any feedback on the general implementation for now?

How do you want to pull in the regexpu dependency? I’m using `require()` as an example in this patch. Let me know if you want me to make any changes to regexpu’s export logic.
We could [Browserify](http://browserify.org/) regexpu and include it in the runtime but that would greatly increase the file size, which is a non-starter IMHO. Do you have an example of ES6 loader being used in Traceur?