BaseTools: add GetMaintainer.py script

BaseTools: add GetMaintainer.py script

Add a new script GetMaintainer.py that uses the new Maintainer.txt format
to determine which addresses to cc on patch submission.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>