shortcut to hide/show controller

If manual control is used we override default youtube autohide behavior
and yield control to the user.

Closes #104, #123.

options.html

@@ -32,6 +32,10 @@
         <label for="fasterKeyInput">Increase speed</label>
         <input id="fasterKeyInput" placeholder="press a key" type="text" value=""/>
       </div>
+      <div class="row">
+        <label for="displayKeyInput">Show/hide controller</label>
+        <input id="displayKeyInput" placeholder="press a key" type="text" value=""/>
+      </div>
     </section>
 
     <section>