colorization

Hi,

I picked up the branch net.venge.monotone.colored-diff, brought it up to
speed (including the newish status output) and adapted the colorization
a bit to .. well .. my taste, I guess.

To argue a bit more objectively: I tried to make the default colors
readable on black as well as white background. And I reduced noise a bit
by un-coloring a couple of things. Colorization can be adjusted via a
lua hook.

I'd opt for enabling colorization by default. Especially as this feature
gets automatically disabled for non-smart terminals. Any objections?

Any specific wishes on how to paint this bike shed?

Regards

Markus Wanner