New css loader plugin

This is a proposal for a new css loader plugin that would inject plain css files instead of js-wrapped ones as it is done today.
It is based on the XStyle's css loader (https://github.com/kriszyp/xstyle/blob/master/core/load-css.js).
The stylesheets are loaded via injected ```<link>``` tags and the plugin will notify the module when the stylesheets have been loaded. Since not all browsers support the ```onload``` event on ```<link>``` elements (concretely, it's all webkit-based browsers with a webkit build <= 535), it also provides an alternate code path to check when the stylesheet is ready using a timeout...

This css loader pluging also contains the code required by the build system to build css layers (cc @clmath).
I understand you are in favor of inlining code as much as possible. My take on this is that functions helps to structure the code, and make it readable, even if they are called once. If the function helps to understand the code, then it should be a sufficient reason to have it (of course, I'm assuming the function does has a added value to exist as a function.).

I would like to understand your position on this. What benefits do you see to inline such a function (I mean when a function is called once like here) ?