Add HPLuv to color picker demo

[boronine]

This has been reported here: #37

And more recently on Twitter.

I want to keep the demo as clean as possible. Perhaps we can add a checkbox like this:

[ ] pastel

[carlinmack]

It's been awhile but agree! I would really like to get X evenly spaced colours from the picker by entering evenly spaced values into the Hue field directly, however this doesn't really look right with the polygonal orbit of HSLuv.

I found that this radius is calculated here https://github.com/hsluv/hsluv/blob/master/website/picker.js#L72 but it's too complex for me to figure out how to modify

[carlinmack]

FWIW now that I know of HPLuv, I was able to get what I wanted with Python :)