Re-add `Rgb` and `Rgba` types, aliased to `Srgb` and `Srgba` types.
Re-add `Rgb` and `Rgba` types, aliased to `Srgb` and `Srgba` types.

Also adds shorthand `rgb` and `rgba` constructors.

This may help new users to get going quickly and avoid stumbling on
color space technicalities they don't really need to know about early
on.

MindBuffer/nannou