Added option for keeping newest duplicate based upon creation time

I have an interesting situation where I'm downloading backups of my emails in Maildir using either mbsync or offlineimap and rsync'ing it somewhere else.  Since I shuffle mail around into different folders I wanted to prevent duplicates by tracking the most recent file/location and dumping all other older duplicate copies of messages.  So.. I added an option '-o, --remove-older' that removes duplicates based upon creation time (as given by os.path.getctime()).  Let me know what you think / if you think it's useful!