csgrep: reformat the list of command-line options
csgrep: improve description of command-line options

... and group the options by purpose, and from most common to useless
csgrep: introduce the --[no-]color program options
color: a separate module for the ColorWriter class
color: take care of handling --[no-]color options
csdiff-core: simplify the signature of diffScans()
csdiff: introduce the --[no-]color program options