Leonardo: add digital pin 7 as an interrupt

[nopdotcom]

(This is tested, but I am not certain about the twisty little maze of pin-to-pin-to-interrupt mappings.)

On Leonardo/32u4, digital pin 7 (aka PE6, aka INT6) is connected to Arduino core interrupt 4. For the Arduino mapping, see:

https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/WInterrupts.c#L102

Tested with this variant on the Basic example:

  Encoder myEnc(7, 6);
  void loop() {
    // demonstrate encoder changes are registered while not being read()
    delay(1000);
    long newPosition = myEnc.read();
    if (newPosition != oldPosition) {
      oldPosition = newPosition;
      Serial.println(newPosition);
    }
  }

I have not removed the “untested” comment on the Leonardo definition, as I have not tested pins 0-3.