Data backup and restore

One of the points of Praefectus is simple data backup and restore. We'll need a simple command that performs a backup, and another one that restores a backup.

When restoring a backup, it should try to merge the data from the backup with the actual data, and also there should be an option to completely destroy the current data and replace it with the backup.

We should take precautions to not lose everything due to the user accidentally performing a descructive backup operation: maybe disallow that on a non-empty database, or ask for additional confirmation, or perform a forced backup before destructive restore.

Depends on:
- [ ] #13 (since we need a UI to add the commands to)
- [ ] #9 (since we need the data to work on)
- [ ] #7 (since we need a data merge algorithm)