Added keyword substitution for user and group emails.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@932034 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep for 4.6.9 release.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@932035 b8457f37-d9ea-0310-8a92-e5e31aec5664