Add the option to synchronize on application close

Closed due to inability to collect 10 votes during more than 2 years. Sorry.

See also http://roadmap.cintanotes.com/topics/578-option-to-start-simplenote-sync-after-any-local-modifications/

Reopened due to new request of the same function

This could be optional or provided only upon an optional prompt ("Would you like to synchronize before exiting?").  It is hard to imagine why someone who has activated sync would not want to sync upon exit, if he or she is actually using the synchronized data.