Support dynamic System.register

[guybedford]

See https://github.com/systemjs/systemjs/blob/master/lib/extension-register.js#L63

When creating bundles for SystemJS, to allow AMD, CommonJS and ES6 to all bundle together we use dynamic System.register with 4 arguments to allow this.

If we could support this form as well in the micro loader, then we would be able to create bundles of any module format and have them work against a micro loader. That would provide a really nice dev -> production workflow with SystemJS -> micro loader.

I'm keen to work along these lines.