fix rgba colors

I had an issue with the script that crashed it on a site with a font color `rgba(0, 0, 0, 0.8)`. This fix should ensure graceful handling of both rgb and rgba definitions.

I was going to test and build it, but I use neither coffee, nor bundler, nor grunt… ;) So this is pretty much uncharted territory for me. After installing all the dependencies, building *without any changes* produces different `.min` that is stored in the repo — I don’t understand why and I don’t want to break anything, but I hope this diff is enough for you to finish it and merge it in.