checkpatch: colorize output to terminal

Add optional colors to make seeing message types a bit easier.
The default is to show them on a tty.

Inspired by Linux commits 57230297116fa ("checkpatch: colorize output
to terminal") and 737c0767758b ("checkpatch: change format of --color
argument to --color[=WHEN]").

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>