Add an "undo" command

[neonrust]

Basically, it just rolls back the database to the most recent backup (i.e. series.1.*).
Can be run multiple times, until there are no more backups left (default 10).

This is probably and mostly only for developer use for debugging purposes. However, it could be useful also for undoing command-line mistakes, e.g. marking the wrong season.

Should of course ask confirmation.