scripts: Teach get_maintainer.pl about the new "R:" tag

We can now designate reviewers in the MAINTAINERS file with the new
"R:" tag, so this commit teaches get_maintainers.pl to add their
email addresses.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>