optionally return last brightness value from Update()

[jandelgado]

an optional int16_t pointer can now be passed to the Update methods, e.g. in Update(int16_t *pLast=nullptr) or Update(uint32_t t, int16_t *pLast=nullptr). If not null, then the last brightness value set on the LED in Update will returned through this pointer. Example usage:

int16_t lastVal = -1;
led.Update(&lastVal);
if (lastVal != -1) {
   // lastVal contains the current brightness value
} 

[jandelgado]

closes #117