Allow link modifications (e.g., to add campaign tracking)

Here's a patch which allows 3rd party modules to modify links in mimemail messages. This makes it possible add tracking code (for example, Google Analytics) to every link in an outgoing email, or any other future uses people might come up with to tweak the links in email (redirect to a different domain, language, etc. )

This patch does 2 things:

- Adds hook_mimemail_url_alter when parsing links in a message, so that other modules can edit these links

- Provides a mechanism to store and retrieve the mailkey, in case this is needed by functions which will later alter the mail (mailkey is passed through initially through 1 or 2 layers of functions, but not available to hooks or other functions which might come into play and which want to alter their behavior depending on the type of email).

And here's some sample code implementing this new hook. This function adds Google Analytics tracking code to all links in the email, using the mailkey as the campaign:

/**
 * Implementation of hook_mimemail_url_alter
 * @param string $url url to alter
 * @return string altered url
 */
function MYMODULE_mimemail_url_alter($url) {
  $campaign = mimemail_key();
  if (!$campaign) $campaign = t("generic");
  $url .= "?utm_source=users&utm_medium=email&utm_campaign=$campaign";
  return $url;
}