Add support for alternative message date synchronisation

Global or per-repository option utime_from_message tells OfflineIMAP
to set file modification time of messages pushed from one repository
to another basing on the message's "Date" header.

This is useful if you are doing some processing/finding on your
Maildir (for example, finding messages older than 3 months),
without parsing each file/message content.

From: Cyril RUSSO <boite.pour.spam@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>