Add TransformableRenderable capability

src/Html/TransformableRenderable.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Erusev\Parsedown\Html;
+
+interface TransformableRenderable extends Renderable
+{
+    /**
+     * Takes a closure $Transform which will provide a transformation of
+     * a "contained text" into Renderables.
+     *
+     * In order for TransformableRenderable to make sense, a Renderable must
+     * have:
+     *   1. Some concept of "contained text". $Transform can be applied
+     *      piece-wise if your container contains logically disjoint sections
+     *      of text.
+     *   2. A generic mechanism for containing other Renderables, or replacing
+     *      the current renderable with a container.
+     *
+     * It is acceptable to only partially transform "contained text".
+     *
+     * @param \Closure(string):Renderable $Transform
+     * @return Renderable
+     */
+    public function transformingContent(\Closure $Transform): Renderable;
+}