#9 hexcompare should refuse to run on a monochrome terminal (or support it plainly)

[Mateusz Viste]

currently hexcompare is unuseable on a monochrome terminal (I learned it the hard way). This is not a big problem, since it's hard to find a non-color terminal these days, nonetheless, hexcompare should address this issue gracefully, that is detect monochrome terminals and either run in a special mono-aware color scheme, or simply tell the user that no color = no fun, to avoid user's frustration.

I attach here a patch that makes hexcompare detect monochrome terminals and output a straight error message telling the user what's wrong.

[Mateusz Viste]

applied.