Better colour parsing

[Remo]

At the moment we only extract shared values for color and font-family, this should be improved.

We should also convert different way to write a colours, uppercase, lowercase and maybe rgb(a) as well as named colours

[lukas-dachtler]

+1 for this feature!
A first step to this large change request could possibly be a "find-and-replace" algorithm for the already extracted variables. In my case, css2less got @color_1 from some color-tag but didn't get 83 other uses of that hex-color hidden somewhere in the large css-file.