Added code for additional color difference formulae

A whole slew of color difference formulae for those who just can't be satisfied with Delta E 2000. ;)

Some have industry applications; some are antique but still used. I will try to look up sources and include them soon.

You'll notice that `colordiff` survived untouched; it is just an alias for `colordiff_2000` now. See line 84.

BFD has a feature to allow giving a white point for the LAB->XYZ and XYZ->LAB transform that happens internally. I left it undocumented to avoid confusion.