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

This allows to deactivate drawing entirely in the example.

examples/draw-features.js

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