patman: add '--get-maintainer-script' argument

This makes it possible to configure a project to use some other
location or script than the default scripts/get_maintainer.pl one used
in the U-Boot and Linux projects. It can be configured via a .patman
configuration file and accepts arguments, as documented.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>