[css-color] Add HCL (aka Lch) color scheme

I propose to add HCL color scheme to CSS.

"The Hue-Chroma-Luminance (HCL) color space is based on how the human perception works. When using HCL, you can directly control the color (hue), the colorness (chroma) and the luminance (brightness).”
– Why HCL? http://hclwizard.org/why-hcl/ <http://hclwizard.org/why-hcl/>


HCL is supported by d3.js:
"D3 supports CIELAB (Lab) and CIELCH (HCL) color spaces, which are designed for humans rather than computers.

Lab and HCL color spaces are special in that the perceived difference between two colors is proportional to their Euclidean distance in color space. This special property, called perceptual uniformity, makes them ideal for accurate visual encoding of data. In contrast, the more familiar RGB and HSL color spaces distort data when used for visualization."
– Mike Bostock, https://gist.github.com/mbostock/3014589 <https://gist.github.com/mbostock/3014589>


HCL color scheme is also available in
* CSS with https://github.com/devgru/postcss-color-hcl <https://github.com/devgru/postcss-color-hcl>
* Chroma.js https://github.com/gka/chroma.js <https://github.com/gka/chroma.js>