Enhancement: increase granularity of the "keep within" retention policy

The `keep-within` option of the `forget` command now accepts time ranges with
an hourly granularity. For example, running `restic forget --keep-within 3d12h`
will keep all the snapshots made within three days and twelve hours from the
time of the latest snapshot.

https://github.com/restic/restic/issues/2089
https://github.com/restic/restic/pull/2090