Add a preview for the selected entity in WYSIWYG

d.o issue: https://drupal.org/node/2280547
It is that only. This placeholder is just the inner HTML since innerHTML can't be empty. Outer HTML is exactly same as the user had entered. For example, `<div data-entity-id="1" data-view-mode="teaser" data-entity-type="node">Placeholder</div>`