Update CLI to enable use for backups

[aled]

Minimal required features:

• backup to SFTP server. Each backup is an immutable filesystem.
• optional verification during backup (entire tree is re-downloaded)
• list backupsets with timestamps
• list history of any file or directory
• restore individual files, or directory trees from any version to any local destination

Unfeatures:

• mutable filesystem
• performance optimisations: multithreading, async, queuing of updates
• local cache of chunk IDs or chunk contents
• deletion of repos, backupsets, backups, files, directories (requires local cache)
• distributed or redundant storage
• include or exclude files/directories from backup

Example commands

• grufs init [--name mystorage] --storage-type sftp --sftp-credentials credentials.json --storage-basedir:~/grufs/storage
• grufs storage list
• grufs backupset init --name mydocuments [--storage mystorage] --directory c:\documents
• grufs backupset list
• grufs backupset log mydocuments
• grufs backup mydocuments
• grufs restore mydocuments --destination c:\restores