[WIP] Twig entity embed

Here's the corresponding d.o issue. https://drupal.org/node/2279257

It is built on 8.x-1.x right now instead of the embed-display branch. This PR will probably stay Work-In-Progress until it uses  EntityEmbedDisplay.
I'm in love with this line. Thank you @stevector for working on this. And thank you @davereid for encouraging him to work on this.