add support for data-visibility=hidden

demo.html

@@ -77,6 +77,13 @@
 					</p>
 				</section>
 
+				<section data-visibility="hidden">
+					<h2>Hidden Slides</h2>
+					<p>
+						This slide is visible in the source, but hidden when the presentation is viewed. You can show all hidden slides by setting the `showHiddenSlides` config option to `true`.
+					</p>
+				</section>
+
 				<section data-auto-animate>
 					<h2 data-id="code-title">Pretty Code</h2>
 					<pre data-id="code-animation"><code class="hljs" data-trim data-line-numbers>