Add --index-only option

[KyrietS]

Option --index-only or --no-copy would just create an index without copying a files.

This index should not be saved like an actual snapshot because it could be used by mistake to create incremental snapshot of files that are not present in the snapshot (because there is only index files)

Or if it is saved like a normal snapshot then I could just check for snapshot integrity before using this snapshot as a base for incremental backup. It makes sense to do it by default because broken base will corrupt all future incremental snapshots.