Feature/enhancement image embeds

The <img>tag which has already existed was also added to the object of interactive.
Your are doubling existing function ```preparePreviousImages()``` which handles exactly the same issue. 

The only difference is, that when you call ```editor.serialize()``` you will get images wrapped in ```<div class="mediumInsert" id="mediumInsert-0"><figure class="mediumInsert-images"></figure></div>```, so ```preparePreviousImages()``` is looking for ```.mediumInsert``` class and not plain ```<img>```.