add HSLuv - Human-friendly HSL

[safareli]

Have not gone into it too much but looks like it could be good alternative to HSL for some applications. On it's website there are couple implementations in different languages and they could be adopted. Not sure if I understand correctly but we then we could also add HSLuv to ColorScale type which could be used in sample function.

http://www.boronine.com/2012/03/26/Color-Spaces-for-Human-Beings/
http://www.boronine.com/2012/10/06/My-Progress-On-HUSL/
https://programmingdesignsystems.com/color/perceptually-uniform-color-spaces/

[sharkdp]

I had the same idea in the past (#12 - the project was called HUSL back then).. not sure why I closed the ticket. Seems like a good fit!

[safareli]

It would have been quite useful for color palette generation, and manipulating colors using code.
I might not be able to implement it shortly but let's have this issue open if someone decides to contribute.