Expose "parseColor" method on style spec package

[lucaswoj]

Users who want to use the function method on the style spec package sometimes need to parse colors too (see https://www.mapbox.com/mapbox-gl-js/example/data-join/). We could make their workflow easier and more consistent by exposing our implementation of parseColor on the style spec package.

cc @kronick @ryanbaumann

[tmcw]

Just exposing this method will probably lead to some unexpected, since it uses a cache. Modifying the output will modify future output for the same input.

parseColor is a pretty thin wrapper around csscolorparser - could we recommend instead that implementors use that module?

[kronick]

@tmcw good catch. Would changing it to return a deep copy of the value from the cache be a bad idea? Do we have a benchmark set up to test the performance impact of a copy? If that isn't a good route, looks like csscolorparser would do most of what we need (save for the counterintuitive math where each RGB component needs to be multiplied by alpha).