Let Plugin-Outlet Render inner template data

 - allows passing of template data to the plugin-outlet to be rendered if no plugin is using that slot
 - acts as a fallback when no plugin is found
 - allows wrapping of existing features inside a plugin-outlet that then plugins can overwrite with their own behaviour
 - backwards compatible
 - uses invisible emberviews (as Ember.Component does)
 - the template has full-access to the controller and view, should be totally transparent
Merge pull request #2449 from ligthyear/plugin-outlet-defaults

Plugin-Outlets-defaults for template overriding