CiviMail - Email Generation

Reduce the number of scans from: 6 + (3 * the number of emails being created/generated) to: 6

As suggested by Shane:
we need to extend the way in which tokens are replaced / managed such that there is a configurable way to add custom tokens so we can avoid adding appreciable overhead to email generation.

File(s) need improvement / modification:
CRM/mailing/BAO/Mailing.php, line 78 (CRM_Utils_Token::replaceSubscribeInviteTokens)