Add support for external piezo buzzers via M300

[chrishamm]

M300 can already send beeps to 12864 buzzers or PanelDue if connected, but configuring a custom external piezo buzzer isn't possible yet. I believe we had this idea ages ago as well.

Maybe M300 could be changed to drive a configured servo/GPIO PWM pin for a given time.

[Billtopia]

I love this idea as I would then be able to put a louder speaker on to my printer. You could have it use a pin set in the object model. state.beep.pin? and set it directly ( unless the state object is read only ) or to better align with the g-code everywhere idea... a m300 c"PinName" by itself to set the pin.