bugfix: colorize output even without discharge rate (thanks f8l)

bugfix: colorize output even without discharge rate (thanks f8l)

On some systems, the discharge rate may be missing, although the battery
is still working. This leads to an edge case on Linux systems in which
the output may not be colorized although a threshold was defined.

This commit fixes the behavior by colorizing output if threshold_type is
set to "percentage". Since we cannot calculate remaining time without
discharge rate, output is still uncolorized in case of threshold_type
set to "time".