Add color difference algorithms

[lehins]

@ekmett mentioned on reddit that he'd like some color distance computation added to Color:

One thing I'd really like to have would be DIN99o color distance computations

It seems like a pretty good idea, I wanna record it before I forget about it. There are sample implementations available in:

• Python: https://github.com/colour-science/colour/blob/master/colour/difference/din99.py
• and JS: https://github.com/Evercoder/d3-color-difference

so it shouldn't be too hard to port it into Haskell.

[JonathanReeve]

This would be really great. I'd get some use out of a Haskell-based Delta-E color distance computation.

[o1lo01ol1o]

Also chiming in for Delta E metric!