(WIP) Loader - incorporate pathogen encoding logic in a separate module under loader.

This is a follow up to https://github.com/yui/yui3/pull/1813. 
I created this pull request specifically for feedback as to the direction that needs to be taken.

@caridy
@juandopazo
@ekashida
I wonder if instead `loader-pathogen-combohandler` should be in by default as part of the `loader` rollup, or just an stand alone module that they can bring in manually as part of the seed, saying `yui/yui.js&loader-pathogen-combohandler/loader-pathogen-combohandler.js`, but I haven't make my mind on this yes, we should talk about it.