support to transpile to `YUI` module system

We will like to have the YUI transpiler included by default, and we (the YUI team) are committed to maintain it.

Another option will be to support extensions somehow, so we can maintain the transpiler in a separate module, but it doesn't seem doable at the moment due to the structure of the code after the last refactor.

TODO: 

* [x] tests
* [x] cli changes
* [x] yui transpiler