Add "None" to draw-features example's select

This allows to deactivate drawing entirely in the example.

examples/draw-features.html

@@ -36,6 +36,7 @@
           <form class="form-inline">
             <label>Geometry type &nbsp;</label>
               <select id="type">
+                <option value="None">None</option>
                 <option value="Point">Point</option>
                 <option value="LineString">LineString</option>
                 <option value="Polygon">Polygon</option>

examples/draw-features.js

@@ -49,11 +49,14 @@ var typeSelect = document.getElementById('type');
 
 var draw; // global so we can remove it later
 function addInteraction() {
+  var value = typeSelect.value;
+  if (value !== 'None') {
     draw = new ol.interaction.Draw({
       source: source,
-    type: /** @type {ol.geom.GeometryType} */ (typeSelect.value)
+      type: /** @type {ol.geom.GeometryType} */ (value)
     });
     map.addInteraction(draw);
+  }
 }