TinyMCE - Customized tags when inserting html (drop block)

It's already possible to drag and drop blocks into the TinyMCE editor - its inserted like this: `div.epi-contentfragment` and replaced by the actual content during rendering.

This doesnt work well for blocks that should be rendered as an inline values. Since its not valid markup to have a <div> inside a <p>, TinyMCE splits the <p> if you try to drop a block as a part of the paragraph - because of the epi-contentfragment.

With the help of event listeners in TinyMCE its possible to rewrite the div.epi-contentfragment to a span.epi-contentfragment but some functionality is lost, for example the edit button because it is queried by this selector: `div.epi-contentfragment:not([mceItem]`

If custom tags were allowed, blocks could be used like inline values.