Digital output control

This adds support for GCode commands M64/M65 which can be useful when adding some customized functionality to a CNC. It currently is only supported on the ATMega 2560 unless you move some pins around on the Arduino Uno in cpu_map.h

This also improves the serial monitor so when a command is typed, it is echoed back to the console followed by an error message if there was any. This allows you to see the history of commands that lead up to a particular point and doesn't leave you guessing as to what was entered if there was an error

Thank you for all the amazing work on grbl!