Add support for all entity D8

Thanks for the initial patches! Commit ba8deeb in the dev branch is optimized similar to what you suggested. A new hook is introduced to make the addtoany extra/pseudo-field available to other entity types. Usage example in a custom module:

/**
 * Implements hook_addtoany_entity_types_alter().
 *
 * @param array $types
 *   The entity types.
 */
function my_module_addtoany_entity_types_alter(&$types) {
  // Add the "taxonomy_term" entity type to make the
  // addtoany extra/pseudo-field available for tags, etc.
  $types[] = 'taxonomy_term';
}

The new hook was favored over new checkboxes in AddToAny settings because we'd like to avoid overcomplicating the options. Patches are welcomed if there are ideas for elegantly exposing the many bundles & entity types in AddToAny settings. Perhaps a form field that autocompletes multiple entity IDs, or some better approach?