Enhancement: Add `--keep-within-hourly` option to restic forget

The `forget` command allowed keeping a given number of hourly
backups or to keep all backups within a given interval, but it
was not possible to specify keeping hourly backups within a given
interval.

The new `--keep-within-hourly` option now offers this functionality.
Similar options for daily/weekly/monthly/yearly are also implemented,
the new options are:

    --keep-within-hourly <1y2m3d4h>
    --keep-within-daily <1y2m3d4h>
    --keep-within-weekly <1y2m3d4h>
    --keep-within-monthly <1y2m3d4h>
    --keep-within-yearly <1y2m3d4h>

https://github.com/restic/restic/issues/3414
https://github.com/restic/restic/pull/3416
https://forum.restic.net/t/forget-policy/4014/11
