Add option to exclude `Date:` from `--checksum` mode

[srcshelton]

I have an account which receives duplicate messages (with differing Message-IDs) a few minutes apart from various mailing-lists. It would be helpful to have an option to exclude the Date: header from checksum calculations to account for these situations.

(Processing a 416-message mailbox found no duplicates with Date: checks enabled, but 106 duplicates with the line commented out!)