new option to hide controller by default

When enabled controller visibility in 'manual' mode: to hide/unhide the
user needs to use the configured shortcut keys.

Closes https://github.com/igrigorik/videospeed/issues/156.

options.html

@@ -36,6 +36,10 @@
         <label for="displayKeyInput">Show/hide controller</label>
         <input id="displayKeyInput" placeholder="press a key" type="text" value=""/>
       </div>
+      <div class="row">
+          <label for="rememberSpeed">Hide controller by default</label>
+          <input id="startHidden" type="checkbox"/>
+      </div>
     </section>
 
     <section>