Implement rdiff-backup support

https://rdiff-backup.net/ is another common backup tool. Implementing the importer might be a bit easier due to the fact that it's also written in python.

I've written a shell script that does it, however it's quite cumbersome as it requires the full extraction of a snapshot for borg to back it up, and rdiff-backup isn't very smart on restore (it restores a file even if it's unchanged).