Add LCH support

[mkalamarz]

LCH is a human-friendlier version of LAB
Wikipedia article: https://en.wikipedia.org/wiki/HCL_color_space
Formula: http://www.brucelindbloom.com/index.html?Eqn_Lab_to_LCH.html

[freekmurze]

That is currently not supported, but feel free to create a PR that adds this functionality. Make sure to update the readme and tests.