plugin hook for adding SASL mechanisms for SMTP

[manu0529]

Until now the only SASL mechanisms that were available for SMTP
authentication were the one Net-SMTP provided. I just had a patch
accepted in Net-SMTP SVN so that new mechanisms can be added on the fly.
One do this by calling the new method:

$conn->setAuthMethod('METHOD_NAME', array($object, $method));

or, without using object programming:

$conn->setAuthMethod('METHOD_NAME', $callback);

In order to use that in RouncCube?, we need a new hook so that plugins
have the opportunity to register their method using
Net_SMTP::setAuthMethod()

Here is the patch that does just that:
​http://ftp.espci.fr/shadow/manu/rcube.diff

[alec]

Extended smtp_connect hook in [3875eb68].