color: disable color when json output is requested

color: disable color when json output is requested

Instead of declaring -color and -json exclusive, ignore -color when
-json is provided. The rationale is to allow to put -color in an alias
for ip while still being able to use -json. -color is merely a
presentation suggestion and we can assume there is nothing to color in
the JSON output.

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>