Colorize terminal messages

Cabal's output is currently all monochrome. It might be nice to make some judicious use of color.

Places that would IMO benefit from color:
- Warnings and errors could be highlighted. I very regularly have colleagues ask for help when the answer is printed out by cabal as a warning, but the warning is swamped by e.g. dependency resolution output, so they don't notice it. Coloring them would help.
- Dependency conflicts. Often there is quite a lot of output explaining why dependency solving failed. Usually I want to pick out the conflicts that forced the solver to discard a branch, but these aren't always easy to see. Again, this is another example where I regularly see people get stuck despite it saying "pkgconfig error: no libX" or whatever - it's just not easy to spot.