Add CPU usage color thresholds

Add CPU usage color thresholds

CPU usage had previously not supported the color option.  Add support
for a "degraded" state above which the degraded color is used, and a
higher "bad" state above which the "bad" color is used.  One possible
use for these might be indicating whether one or all cores are
saturated.

Unlike the color settings for other, these are set high enough to be
disabled by default.  This is done because i3status determines CPU
usage over only the last display interval, which means that, a user
with a low refresh rate might see frequent, potentially-annoying color
changes.