Add support for HWB

[dblock]

https://en.wikipedia.org/wiki/HWB_color_model

HWB is a cylindrical-coordinate representation of points in an RGB color model, similar to HSL and HSV.

[thisandagain]

HWB is an awesome color system ... super intuitive. Major 👍 to this.

[dblock]

It was suggested by @LeaVerou, btw.