Add functions for blending and overlaying

[AshleyYakeley]

I'm not sure if there's already an easy way to do this, but it would be nice to have some of the same functionality that colour has. Something like:

• blend :: (...) => e -> Color cs e -> Color cs e -> Color cs e
• over :: (...) => Color (Alpha cs) e -> Color cs e -> Color cs e
• overAlpha :: (...) => Color (Alpha cs) e -> Color (Alpha cs) e -> Color (Alpha cs) e

As I understand it, in colour, blends are always done in linear space.

[AshleyYakeley]

Is this project still active?

[lehins]

Project is alive. If someone will submit a PR I am not gonna let it sit. Right this moment in life I just don't have much time to work on it myself, unfortunately.