Rework color variants in order to reduce file size

[samanpwbb]

Our best bet for cutting down on Assembly's built file size is to rework the color variant build process. We generate a lot of CSS in order to support multi-color forms. Most apps only need one checkbox color, but often apps will want a few different button colors. Can the color variant API be adjusted so user may declare which form elements they'd like to color which colors?

[samanpwbb]

The option to do this already exists, we just haven't set it up (see https://github.com/mapbox/assembly#colorvariants-option).