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

Restic forget allowed users to specify keeping a given number of
hourly backups, or to keep all backups within a given interval,
but not both, that is to specify keeping hourly backups within
a given interval.

The `--keep-within-hourly` option offers this functionality and
similar options for daily/weekly/monthly/yearly are also
implemented. The new flags 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