Keep folder structure [Feature Request]

[flungo]

Currently it would seem that when you use buttersink it will recurse through the source directory looking for any snapshot at any depth within the source and then output these as a flat hierarchy into the destination. It would be nice if there was a --keep-structure flag that would keep the structure relative to the source when copying to the destination. Where folders don't exist in the destination they can be created and where there are subvolumes within the source it might also be useful to have a --keep-subvolumes where it will create a subvolume (rather than an ordinary folder) to match the source (only applicable where the destination is btrfs).