Add no-close option to leave deleted messages

On a dovecot server, the messages get deleted when imapdedup calls
close. However, in order to review the delete flags on a mail client
it is convenient to have them just flagged, so the user can expunge
them manually.
Store message only in verbose mode

To save memory, we should store the message only in verbose mode.
This allows to process 0.5M messages without the need of 4GB of
memmory.
Update help text and README.
Merge branch 'falstaff84-master'