Provide a per-storage init command

[nir0s]

Both the Consul and Vault (and later Elasticsearch, etc..) storage backends can receive additional configuration options currently not exposed via the CLI. The API comfortably exposes those options.

Creating a command which will allow to initialize each specific storage will make configuring the storage easier:

ghost init consul --stash http://localhost:8500 --directory 'my_dir' --verify --client-cert 'my_cert' --auth user:password

An alternative could be to allow the user to pass any set of kwargs to the init command like so:

ghost init http://localhost:8500 --backend consul --storage-args directory=mydir;verify=true;client_cert=my_cert;auth=user:password