Enhancement: Add directory filter to ls command

The ls command can now be filtered by directories, so that only files in the
given directories will be shown. If the --recursive flag is specified, then
ls will traverse subfolders and list their files as well.

It used to be possible to specify multiple snapshots, but that has been
replaced by only one snapshot and the possibility of specifying multiple
directories.

Specifying directories constrains the walk, which can significantly speed up
the listing.

https://github.com/restic/restic/issues/1940
https://github.com/restic/restic/pull/1941
