Enhancement: Add dirs `tags` and `hosts` to fuse mount

The fuse mount now has two more directories: `tags` contains a subdir for
each tag, which in turn contains only the snapshots that have this tag. The
subdir `hosts` contains a subdir for each host that has a snapshot, and the
subdir contains the snapshots for that host.

https://github.com/restic/restic/issues/636
https://github.com/restic/restic/pull/1050
