Add addEventListener() and removeEventListener() methods

Currently event handlers can only be setup at initialization time in the constructor's options parameter.

Instead, iFrameResizer should implement the standard EventTarget interface

• https://developer.mozilla.org/en-US/docs/Web/API/EventTarget

with addEventListener(), removeEventListener(), and dispatchEvent() so 1) iFrameResizer follows the standard JS events API and 2) event handling can change dynamically.