Retain days, weeks, months, years

[twiddler]

I like to keep many backups of the last few days, but only a few of the last years. restic supports retaining a different number of backups for the last days, weeks, months and years via --keep-daily, --keep-weekly, --keep-monthly and --keep-yearly. The app currently seems to only support --keep-daily. It'd be great if you could also add the other options. 🙏