Most terminal emulators support a palette of 216 indexed colors:
the 6x6x6 color cube pioneered by Xterm. But as an index gives
little indication about what kind of color it produces, allow the
user to specify the color with three hex digits instead, one for
the level of red, green, and blue each, in the format #rgb.
One hex digit allows for sixteen values, but as there are only
six levels available, only 0, 4, 8, a, c, and e are significant
-- other values get reduced to the nearest lower one.
This fulfills https://savannah.gnu.org/bugs/?56445.
Requested-by: Peter Liscovius, Erik Lundin, Owen Maresh, Dave Lemonby
Signed-off-by: Brad Town <brad@bradtown.com>
8d6b468cdc