Make the IE11 addEventListener polyfill optional

[buschtoens]

IE11 has an incomplete and buggy implementation of Element#addEventListener, which is why I have added event-listener.js in #2 to make this addon just work^™️ in IE11.

This polyfill is of course not required, if the targeted browsers do not include IE11.

We should add a config option to allow excluding the polyfill from the build. Ideally this option would by default depend on the support for { once: true } in the browsers defined in config/tragets.js, so that users don't need to configure this manually.

[buschtoens]

The polyfill now also adds a hard assertion against calling event.preventDefault() for passive event listeners. If the polyfill may be excluded, this assertion has to be moved.