Ability to perform custom processing when receiving emails

Feature committed.

To document, the new hook is support_fetch_message_alter.

A silly example use case, in a module named 'sample':

sample_support_fetch_message_alter(&$message, $client) {
  if ($message['from'] == 'nobody@nowhere.net') {
    $message['body'] .= "\nThis text was appended to the message body.\n"
  }
}