Add optional argument to onRun #27

It would be great to be able to add an optional void (*arg) argument to onRun and have that passed to your callback.

(poorly written) example

void shutoff(void (*arg)) {
  uint8_t *pin = (uint8_t*)arg;
  digitalWrite(pin, LOW);
}

void setup() {
  Thread t1 = Thread();
  t1.onRun(shutoff, 1);

  Thread t1 = Thread();
  t1.onRun(shutoff, 2);
}

This will dramatically simplify code by not requiring individual callbacks for each thread that perform the same function.