Digital output control (M64/M65)

This adds support for GCode commands M64/M65 which can be useful when adding some customized functionality to a CNC. An example use case: I have a laser guide setup along the axises of the drill. The M64/M65 commands can be used to turn on and off the lasers on their pin when we use them to setup an object on the plate of the CNC. This allows for easy addition of small features to your CNC to be controllable via these commands.

It currently is only supported on the ATMega 2560 unless you exchange some pins for the Arduino Uno in cpu_map.h.

Thank you for all the amazing work on grbl!