def default_command(command_name = nil)
      if command_name
        if commands.key?(command_name)
          @default_command = commands[command_name] if command_name
          @default_command
        else
          raise ArgumentError, "'#{command_name}' couldn't be found in this command's list of commands."
        end
      else
        @default_command
      end
    end
def default_command(command_name = nil)
      if command_name
        if commands.key?(command_name)
          @default_command = commands[command_name] if command_name
          @default_command
        else
          raise ArgumentError, "'#{command_name}' couldn't be found in this command's list of commands."
        end
      else
        @default_command
      end
    end
Public: Sets the default command

 command_name - the command name to be executed in the event no args are
                present

 Returns the default command if there is one, `nil` otherwise