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.
Well not that it matters much in practice, but the issue isn't 'no way of using the npm version' it's just we have no way currently.  We would need a way to convert npm modules to our es6 module format or to global scripts we can concat with our modules. I think it would be ok to have the build process read out of `node_modules`.