SystemJS support

[sebmck]

Currently esperanto can't compile to SystemJS. It's the only module loader that supports circular dependencies, live bindings etc. Without these it's impossible to replicate ES6 modules exactly.

Reference: https://github.com/ModuleLoader/es6-module-loader/wiki/System.register-Explained

[Rich-Harris]

Thanks for the link. I hadn't really looked into it - in my head SystemJS was kinda like AMD except with System.register instead of define...

Agree that it would be worth supporting.

[eventualbuddha]

As mentioned in #51 a bundle of ES6 modules can also support ES6 module spec fully. That said, SystemJS would be good to support.

EDIT: And by "fully" I mean "as far as static imports and exports are concerned".

[Rich-Harris]

Closing this in favour of rollup/rollup#83, as I'm planning to abandon Esperanto in favour of Rollup. For one-to-one transformations I'm recommending people use Babel