Add Display plugin options

d.o issue: https://drupal.org/node/2283691

When this issue goes through, following follow-up issues will be needed:

* Remove redundant 'view-mode' field from the form.
* Use rebuildEmbedForm() in submitForm() when form['state'] is select.
I'm not sure this is necessary? In my quick testing, I think when saved it should escape double quotes to a `&quot;` character in the JSON? Or is that not happening?