RYB color model support

[iblislin]

• https://en.wikipedia.org/wiki/RYB_color_model
• Algorithm for coverting between RYB and RGB is here:
  There is example code at the appendix A.
  https://github.com/bahamas10/ryb/raw/gh-pages/assets/ryb.pdf

[timholy]

Needs a volunteer to implement this.

[kimikage]

I personally recommend that when you add a new color type, you first implement it as a separate package and make sure that the ecosystem such as JuliaImages is compatible with it. Then, if there is a need or demand for the new type, I think it would be good to incorporate it into ColorTypes.jl and Colors.jl.

I plan to create a tutorial on implementing custom color types after the release of ColorTypes v0.12.
(cf. JuliaGraphics/ColorTypes.jl#217)

I also have a plan to support the CMY color as a special case of CMYK. However, it still takes time to support CMYK colors, as we practically have to wait for the support for color profiles.

If you need exactly this RYB model instead of CMY, please emphasize the needs.