Adding and removing event listeners

[stylesuxx]

In the "real" Web Serial API it is possible to add and remove Event listener that listen on the connect and disconnect events:

serial.addEventListener('connect', connectHandler);
serial.addEventListener('disconnect', disconnectHandler);

The disconnect listener is IMHO especially important, so that one can clean up after the device has been disconnected.

[stylesuxx]

@reillyeon I just checked - navigator.usb also allows to add and remove listeners, so this could just be passed through from what I understand.

If you like, I can implement that and submit a PR, I am just asking since I saw you already fixed #19 yesterday and thought maybe you have the listener changes stashed away and ready to submit ;-)