led change notify

[m-mcgowan]

This change addressses #302.

User code can implement a function

void onChangeRGBLED(uint8_t r, uint8_t g, uint8_t b) {
// do stuff with the LED values
}

Which is called any time the LED changes color (either due to user code calling RGB.color() or the system updating the LED, e.g. breathing cyan.)

For example, this could be used to use analogWrite() to set PWM outputs to an external RGB led:

void onChangeRGBLED(uint8_t r, uint8_t g, uint8_t b) {
   analogWrite(D0, r);
   analogWrite(D1, g);
   analogWrite(D2, b);
}

[m-mcgowan]

Refactored this and included for 0.4.0.