Replace initialState by a placeholder in slide and code

www/code/inner.html

@@ -30,6 +30,7 @@
     <script src="/bower_components/codemirror/addon/fold/xml-fold.js"></script>
     <script src="/bower_components/codemirror/addon/fold/markdown-fold.js"></script>
     <script src="/bower_components/codemirror/addon/fold/comment-fold.js"></script>
+    <script src="/bower_components/codemirror/addon/display/placeholder.js"></script>
     <style>
       html, body{
           height: 100%;