#261 Mailman should detect mailextensions

[Peer Heinlein]

Mailextensions are realised by adding "+extension" to the userpart of the e-mail-address. It's helpful to filter mailinglists and other stuff.

p.heinlein+extension1@heinlein-support.de is the same like "p.heinlein@heinlein-support.de".

If user+extension@ is subscribed to a mailinglist, Mailman should also accept user@ as a subscribed member address.