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.
Only the 'entity' value actually needs to be passed in using setContextValue(). The rest should be passed in using setAttributes($array).