Back up review file before upload

[rmloveland]

This change does the dumbest thing possible: make a backup of the review file with a .bak extension before attempting to push it back up to Github.

This addresses the main concern expressed in #10, which is that under the current behavior if the API call to Github fails, the temp file still gets deleted, and you can lose a lot of work.

[jordanlewis]

Will this lead to a lot of bak files everywhere that don't get cleaned up? :)

If you add a thing that deletes the bak file after a successful push that would feel more appealing to me if you wouldn't mind!