Enhancement: Allow keeping a time range of snapshots

We've added the `--keep-within` option to the `forget` command. It instructs
restic to keep all snapshots within the given duration since the newest
snapshot. For example, running `restic forget --keep-within 5m7d` will keep all
snapshots which have been made in the five months and seven days since the
latest snapshot.

https://github.com/restic/restic/pull/1735
