Add state renderable -- all renderables are state renderables

src/AST/StateRenderable.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Erusev\Parsedown\AST;
+
+use Erusev\Parsedown\Html\Renderable;
+use Erusev\Parsedown\State;
+
+interface StateRenderable
+{
+    /**
+     * @param State $State
+     * @return Renderable
+     */
+    public function renderable(State $State);
+}

src/Html/Renderable.php

@@ -2,7 +2,9 @@
 
 namespace Erusev\Parsedown\Html;
 
-interface Renderable
+use Erusev\Parsedown\AST\StateRenderable;
+
+interface Renderable extends StateRenderable
 {
     /** @return string */
     public function getHtml();