Allow one plan to specify multiple repositories

[yatesco]

I often want to "restic away" my files to multiple repositories. I know restic itself has support for offsite replication of an existing repository (even if I can't find the link anymore!), but sometimes I'd rather do the same restic job twice, for peace of mind.

It would be great if when creating a plan I could:

• choose multiple repositories
• for each repository choose to use restic's replication or choose to do them "independently" (i.e. two parallel restic jobs)

The plan should treat updating all of the repositories as a single atomic action, i.e. it passes if, and only if all are updated.