Needed Filestore Command For Basic Implementation

Relates to https://github.com/ipfs/go-ipfs/pull/3629

Fundamental:

* `ls` - list contents of filestore
* `verify` - verify blocks in filestore
* `rm` - remove blocks from just the filestore - either than or enhance `block rm` to support removing blocks from either the filestore or the main store. In either case is important that pinned blocks should be allowed to be removed from one store as long as they are also in the other
* `dups` - list blocks both in the main store in the filestore.

Nice to have:

* `mv` move a block into the filestore. That is copy the block into the filesystem and add it to the filestore without having to rehash it.