Custom CSS Generation

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