Abstract entity rendering with EntityEmbedDisplay.

I think this is how I intend to solve https://drupal.org/node/2273393. With an abstract EntityEmbedDisplay, I could add sub-classes for Entity Reference formatters, or other field formatters. This would allow users to create arbitrary ways to display embedded entities as well outside of just entity_view(), which will be very useful.

Tasks:
- [ ] Convert to plugins
- [ ] Figure out if derivative plugins can work somehow