imap: Search for unseen messages, mark processed messages as seen

Previously the IMAP receiver deleted messages that were processed. This is drastic in some situations, so we default to marking the messages as seen.

To restore the previous deletion behavior, set the done_flags option:

    Mailman.config.imap = {
      # ...
      done_flags: [Net::IMAP::DELETED]
    }

Closes #64