Add --color=auto option, and make it the default.

Like git, --color=auto turns on color output if the output device is a TTY,
and disables it otherwise.  You can still force color output with
--color or --no-color (or --color=false).  Use '--color' in runtests.sh
to make current (eccentric) behavior.

Change-Id: I1b2638b1f8973a4f8d92f5e1dca91a9ce8220c46