mail collector: Add option to delete old emails

[gethvi]

As we are running the IntelMQ for quite some time now, the mailbox gets really cluttered. It would be nice if the mail collector could also remove old emails. This could be in a form of a now configuration parameter such as delete_older_than. It would delete only old emails matching it's subject regex.

[gutsohnCERT]

Hey!

Really good idea! I am working on it :)