New option to use To header when BCC limit set to 1 added.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@873970 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep for 4.6.6 release.

git-svn-id: https://plugins.svn.wordpress.org/email-users/trunk@873971 b8457f37-d9ea-0310-8a92-e5e31aec5664