Add ability to reverse and slice gradients

[mezarque]

Gradients should have the ability to be reversed, and reversed gradients should also be registered to matplotlib.

In addition, it would be nice to cut a gradient off at ~80%, for example, and return a new, re-jiggered gradient.

Related to this, it would be cool to have functionality where users can provide a list of colors and a gradient with linearly increasing lightness is automatically generated, with the positions along the gradient being determined by the lightness.

[mezarque]

Gradient reversal is resolved by #20; slicing not yet supported.