Working label to X-Keywords sync support gated by config option

This PR supports syncing X-Keywords with labels in Sup.  It requires the offlineimap fork available here: https://github.com/aroig/offlineimap.  A PR against the official offlineimap repo has been sent but not accepted yet.

The config option is `sync_labels_to_xkeywords`.  When set True email messages in the Maildir will be rewritten to add the X-Keywords header.  When set to False no mail will be rewritten.