Add option to skip htmlmail from mail template

We needed a simple and consistent way to bypass the htmlmail (and just send default plain text email). By wrapping the alter hook code, and checking for a variable $message['no_htmlmail'] we make it possible to skip htmlmail if the $message['no_htmlmail']) parameter is set in any email template.

function htmlmail_mail_alter(&$message) {
  // Skip htmlmail if no_htmlmail property is set
  if (!isset($message['no_htmlmail'])) {
    [original code goes here]
  }
}

This will not have any effect on existing installations, so please consider the solution and add it to the module if you think it's a nice feature...