Add support for multiple level CLI commands

E.g. `docker groups create` will attempt to call the function `CmdGroupsCreate` on `DockerCli`. Also, `docker help groups create` behaves the same as `docker groups create --help`.

[Works a bit like this.](https://github.com/bfirsh/docker/blob/ba7d30c27389e0ad44f9cfe505eb72df97f626c0/api/client/commands.go#L2504-L2725)