HCG color model suggestion

Hello. I suggest add support for HCG color model.
https://github.com/acterhd/hcg-color

[seven-phases-max]

Previously it was less or more agreed that the Less core just can't support all of tens of colors models used out there. One can easily add whatever color model support via plugin. See #1853 for more details.

[seven-phases-max]

Merging to discussion at #1853.