Media type plugins

Initial implementation. Should be ready for review. Field map UI is a follow-up (https://www.drupal.org/node/2336157).

https://www.drupal.org/node/2335645

Also done as part of this (test implementation): https://github.com/drupal-media/media_entity_youtube
Hm... would be nice but merging those two loops would probably result in some strange code (defining #options after form element definition, ....). I am afraid doing that would actually result in less readable code. 

In terms of performance it shouldn't have any significant impact as it is O(n) in both cases. :)