Oleksii

Memory leak in contentWindow addImageListeners

1

function [addImageListeners](https://github.com/davidjbradshaw/iframe-resizer/blob/master/src/iframeResizer.contentWindow.js#L744) can be called multiple times for same image, so image element can be [pushed](https://github.com/davidjbradshaw/iframe-resizer/blob/master/src/iframeResizer.contentWindow.js#L749) multiple times, but calling [element.addEventListener](https://github.com/davidjbradshaw/iframe-resizer/blob/master/src/iframeResizer.contentWindow.js#L747) load/error with same handler multiple times adds handler only...