Add convenience gradients

[awelkie]

Just like you have named colors, it would be nice to have a collection of predefined gradients.

For reference:
http://matplotlib.org/xkcd/examples/color/colormaps_reference.html
https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/_cm.py

[Ogeon]

Sure, why not? Some of those are pretty standard, so I guess they will go well with the color constants. Thanks for the idea 👍

[Ogeon]

Removed the "easy / new contributor" label because I underestimated the possibly required amount of Rust knowledge and complexity of the task. It's probably not a super hard problem, just not something that fulfills the criteria of the label.