Make output coloring optional

[kasimon]

I'm having a cosmetic issue: When run on a black-on-white terminal, at least the yellow output is nearly complete unreadable and I have to copy-and-paste it to be able to actually read it. Please either provide a switch to deactivate coloring or detect whether the output goes to a terminal and show colors only then, so that './bashcheck | cat' removes the colors, like ls does.