Metatag settings cannot be set for each views display separately

Short description: Metatag settings cannot be set for each views display (page) separately.

Longer description:

1. In order for a plugin to be in a default or overridden state, plugin extender should implement defaultable_sections() method. Currently metatag_views_plugin_display_extender_metatags class does not have such method therefore user cannot select overridden state for Metatag settings.
2. In order for Views to know whether current display has default or overridden values, [defaults][metatag] => array() should be defined in display options (see Views/includes/admin.inc). This seemingly cannot be achieved by options_definition_alter() method, therefore these options have to be bluntly set in init() method of metatag_views_plugin_display_extender_metatags class.

I'm leaning towards bug report rather than feature request because each page display can have unique page title but currently cannot have unique metatag settings per display.