Support for Quote-less urls in CSS

Hi there,
  I really like your task here, i was going to have to write a similar one; but had the good graces to find your project :+1: 

I have a particular project which i use grunt-contrib-cssmin, which uses cleancss; in the course of the minification process css urls are stripped of their quotes, I'm assuming to save space.

It is valid syntax for css urls to have no quotes:
  http://www.w3.org/TR/CSS2/syndata.html#uri

I'd like to purpose this fix to be able to catch such urls, allowing for quote-less urls in css. I've already tested this on my branch.

Thanks in advance for the review,
Cody