refactor(accordion): add key navigation to accordion

demo/src/app/components/+accordion/demos/custom-html/custom-html.html

@@ -1,15 +1,15 @@
 <accordion>
   <accordion-group>
-    <div accordion-heading class="clearfix">
-      I can have markup!
+    <button class="btn btn-link btn-block clearfix" accordion-heading>
+      <div class="pull-left float-left">I can have markup!</div>
       <span class="badge badge-secondary float-right pull-right">Some HTML here</span>
-    </div>
+    </button>
     This is just some content to illustrate fancy headings.
   </accordion-group>
   <accordion-group>
-    <div accordion-heading>
+    <button class="btn btn-link" accordion-heading>
       I can have markup, too!
-    </div>
+    </button>
     <span class="badge badge-secondary center">And some HTML here</span>
   </accordion-group>
 </accordion>