tumblr_backup: --continue and related new behavior

Included revisions:
- Allow setting options in BACKUP_CHANGING_OPTIONS to match the backup
- Fix incorrectly sorted imports
- Use lists in a few places for consistency with JSON
- Set before to oldest_tstamp even without --period
- Fix argument check to allow --auto 0
- Pass all path parts to open_text in save_post to ensure mkdir

Fixes bbolli#115