Add test image for color quantization

[adrhill]

This would be useful to compare color algorithms in the docs.
Alternatively, the image could also be added to TestImages.jl.

Examples

• The ImageMagick colorwheel:
  

• Images used by Joel Yliluoma (Appendix 2-3):
  
  

[johnnychen94]

@kimkage Does Colors.jl already has the utilities to build this colorwheel image? These look like projection maps from the color spherical onto the 2d space.

[johnnychen94]

Thoughts on JuliaGraphics/Colors.jl#520? I used it in recent ImageInTerminal refactoring JuliaImages/ImageInTerminal.jl#62 and it's working well.

[adrhill]

Love it!