Enhancement: Add --keep-within-hourly switch 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 switch offers this functionality, and 
parallell switches for daily/weekly/monthly/yearly are also 
implemneted. The new switches 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