Allow to trigger callback after an effect is finished

[jandelgado]

#82 inspired this MR:
Added a new UpdateAndFinally method that will update the effect like Update but will trigger the provided callback when the effect is finished. This allows e.g. reconfiguration of an effect.

Example:

#include <jled.h>

auto breathe = JLed(2).Breathe(500);

void setup() { }

void loop()  {
  breathe.UpdateAndFinally(
          [] (JLed& led, void*) {
            led.Breathe(random(50,5000));
            led.Reset();
    }
  );
}

[coveralls]

Coverage increased (+0.08%) to 97.03% when pulling 12684d8 on add_update_and_finally into 1971249 on master.