allow to disable serial debug (from outside)

[Westworld]

While you allow to disable serial debug, the library set by default:
#define SERIAL_DEBUG

inside the library code. Disabling serial debug require to modify the library source itself, it would be nice if there would be an external flag to disable the automatic enabling...

Even when serial debug is disabled, the library returns twice a 0, because it uses:
Serial.println(mirror_x);
Serial.println(mirror_y);
without checking for SERIAL_DEBUG.