LearnBoost/stylus — pull request #559 by mekwall (2012-02-08 10:21:35+00:00)
Title: Implemented the built-in invert as per #558

Inverts the color. The red, green, and blue values are inverted, while the opacity is left alone.

Reference: [SASS documentation](http://sass-lang.com/docs/yardoc/Sass/Script/Functions.html#invert-instance_method)