HCG color model manipulators

[ghost]

These manipulators for those, who will making dark and light themes, without color changes itself (doesn't change hue and chroma value). It useful for invert light into dark themes, and revert. Added 3 operators: lightenHcg, darkenHcg and negateHcg (now tinten, shaden, negateTones).

.light-theme {
  $mod: 0; /* dark font */
  color: hcg(240, 50, $mod);
}

.dark-theme {
  $mod: 100; /* light font */
  color: hcg(240, 50, $mod);
  filter: invert(100%) hue-rotate(180deg); /* same effect */ 
}

[Qix-]

Yeah the naming is exactly why I've not been accepting PRs for a long while. How to go about dealing with this problem is still a mystery to me.

The last thing I want is to dilute the API. Nobody is going to be able to read code and see "tintify" and know what it does.

I appreciate the PR and I'm sure it'll get accepted, just not right now. I still have to come up with a solution to this problem.