Adds LAB and HCL color spaces

[gampleman]

This PR adds the math to specify and output colors in the CIELAB and HCL color spaces.

These are especially useful for information visualization as they are perceptually uniform, but are also suitable for color manipulation and UI tasks, like generating palettes or themes.

There are some open API design questions/caveats:

• [ ] I didn't include convenience constructors. Seems like these are somewhat less common, so having just the record based thing seemed fine. But it is somewhat inconsistent with the others.
• [ ] The coordinates are not normalized like the others. I've not seen them used normalized anywhere, so would consider that strange (also the ranges are not limited like RGB, but are more "recommended", so normalization is somewhat subjective).
• [ ] As there is a lot more computation, there is considerably more rounding error. I think this is fine.
• [ ] Lab's channels don't really have human names. They are called L*, a* and b*. This makes the record a bit awkward...
• [ ] Hcl in the reference implementation I was following explicitly uses NaN values for hue and chroma for black, white, and NaN chroma for grays.

I would be grateful for direction on the above points.

[avh4]

Thanks for taking the time to put this together, and sorry for the slow reply.

My current goal for this package is to be a minimal API that allows other packages to have a common Color type that they can all use. With that goal in mind, I'm avoiding add all possible colorspace conversions and trying to keep this package as small as reasonably possible.

Would you be interested in making a separate package for these additional color space conversions? If you do publish one, I'd be happy to link to it in the README for this package.

In any case, I'll leave this PR open for further discussion and for reference.