docs: Add documentation for --color-* options in markdown and manpage

It took me way too long to find these options, and I was seriously
considering implementing them myself when I found the issue on GH and
the commit closing it.

I just copied the texts from the built-in usage function and modified the
format somewhat to conform to that used in the man page/MD.
Merge pull request #253 from catharsis/color-docs

docs: Add documentation for --color-* options in markdown and manpage