benchmark_color: fix auto option (#559) (#699)

As prevously written, "--benchmark_color=auto" was treated as true,
because IsTruthyFlagValue("auto") returned true.  The fix is to
rely on IsColorTerminal test only if the flag value is "auto",
and fall back to IsTruthyFlagValue otherwise.  I also integrated
force_no_color check into the same block.