Add optional callbacks to the command behaviour

deps/rabbitmq_cli/lib/rabbitmq/cli/command_behaviour.ex

@@ -25,5 +25,13 @@ defmodule RabbitMQ.CLI.CommandBehaviour do
   @callback aliases() :: Keyword.t
   # Coerces run/2 return value into the standard command output form
   # that is then formatted, printed and returned as an exit code.
-  @callback output(any, Map.t) :: :ok | {:ok, any} | {:stream, Enum.t} | {:error, ExitCodes.exit_code, [String.t]}
+  # There is a default implementation for this callback in DefaultOutput module
+  @callback output(any, Map.t) :: :ok | {:ok, any} | {:stream, Enum.t} |
+                                  {:error, ExitCodes.exit_code, [String.t]}
+  @optional_callbacks formatter: 0,
+                      scopes: 0,
+                      usage_additional: 0
+  @callback formatter() :: Atom.t
+  @callback scopes() :: [Atom.t]
+  @callback usage_additional() :: String.t | [String.t]
 end