image&font support + asset structure handling with proper css rewrite + code cleanup

[tomgrv]

I added image support, with the possibility to inline small images in css file as data uri, in order to avoid image folder manipulation in gulpfile and/or prevent image overwriting between libraries

[EHLOVader]

Thank you for this. :+1: Exactly what I needed for Select2 and the sprites.

EDIT: Almost what I needed...

I don't want the CSS files or JS files because I am using useref. However that doesn't handle all the fonts and images. I forked and will update documentation and pullrequest...

Question though, should we all be making pull requests based on others pull requests like this?

[tomgrv]

Cumulative update with @loranger & @EHLOVader contributions Add image support Code cleanup (notification, modules) Add option for assets structure handling