Auto merge of servo#323 - tiaanl:abstract-parse, r=emilio

Refactor color parser to use a generic as output

This refactor allows the color parser to construct new color values directly, in stead of constructing a predefined color value and then forcing lib users to convert that type into whichever format they want to use in the end.

Most common use case is that the color included in the lib, cssparser::Color, is good for serialization, but not well suited for doing calculations on.

Author: bors-servo