Add option to specify which day the weekly backup should cover

[vilitux]

Would be nice to be able to specify on which day the weekly backup should be created (or, as bonus, time?). This can be nice in combination with other backup strategies for e.g. MariaDB, where we do weekly backup of databases on sunday (+ binlogs every day to cover the whole week).

Creating a weekly backup in planb on saturday (or sunday before mariadb backup if time option = yes), would then give us the mariabackup + binlogs till saturday/sunday.