Add toggle command (mpc like)

[dan-cristian]

mpc like toggle command seems to be unsupported, would be a good addition.
"mpc toggle Toggles Play/Pause, plays if stopped"

[LevitatingBusinessMan]

Extend the class like this:

class MPD
      # Play if paused, pause if playing
      # @macro returnraise
      def toggle
        case self.status[:state]
        when :play
          send_command :pause
        else
          send_command :play
        end
      end
end