show labels for brush width

www/whiteboard/index.html

@@ -75,7 +75,7 @@
 
     <div id="controls">
         <button id="clear">Clear</button>
-        <input id="width" type="number" value="5"></input>
+        <input id="width" type="range" value="5" min="1" max="100"></input><label for="width">5</label>
         <div id="colors">&nbsp;</div>
     </div>