Custom CSS Generation

This PR enables a function to be passed instead of the namespace string that, allowing a completely custom output rule set.

The image object is passed to the user's function, which must return a CSS rule set.

I want this functionality in order to [provide a fallback for SVG's](http://germanforblack.com/post/43975575422/protip-all-browsers-that-support-svg-background-images).

Thanks!
Ash