Merged in feature/from-pantheon (pull request #16)

code from pantheon

* code from pantheon

wp/wp-content/plugins/wordpress-seo/src/initializers/initializer-interface.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Yoast\WP\SEO\Initializers;
+
+use Yoast\WP\SEO\Loadable_Interface;
+
+/**
+ * Integration interface definition.
+ *
+ * An interface for registering integrations with WordPress.
+ */
+interface Initializer_Interface extends Loadable_Interface {
+
+	/**
+	 * Runs this initializer.
+	 *
+	 * @return void
+	 */
+	public function initialize();
+}