Implement color functions.

[back2dos]

• [x] mix(color1, color2, factor = .5) (apparently this is done in RGB space)
• [x] fade(color, by) / opacity(color, to)
• [x] saturate(color, by) / saturation(color, to)
• [ ] lighten(color, by) / lightness(color, to)
• [x] huerotate(color, by) / hue(color, to)
• [ ] contrast(color, dark = black, light = white, threshold = 50%)
• [x] invert(color)

[back2dos]

Probably also color blending functions: http://lesscss.org/functions/#color-blending