- Improve finding backup from another interval:
   o strip of interval name
   o run sort -n
   o use the last entry
- do not delete_incomplete, when there's only one backup left!
- Add filter support
   * filter
- think about 'prefer_same_interval' / 'prefer_latest'
- add --version, -V
- add source/ignore_failed_pre_exec
- add source/ignore_failed_post_exec