Add ol.interaction.Draw#extend

This commit adds an `extend` method to the draw interaction. This method makes it possible to extend (continue drawing) an existing line string feature. It only works on line string features, where this functionality makes the most sense.

Here's an example on how to use `extend`:

```js
var vectorSource = vectorLayer.getSource();
vectroSource.removeFeature(feature);
drawInteraction.extend(feature);
```

test/spec/ol/interaction/drawinteraction.test.js

@@ -781,12 +781,43 @@ describe('ol.interaction.Draw', function() {
       expect(coordinates[0][0][1]).to.roughlyEqual(20, 1e-9);
     });
   });
+
+  describe('extend an existing feature', function() {
+    var draw;
+    var feature;
+
+    beforeEach(function() {
+      draw = new ol.interaction.Draw({
+        source: source,
+        type: ol.geom.GeometryType.LINE_STRING
+      });
+      map.addInteraction(draw);
+      feature = new ol.Feature(
+          new ol.geom.LineString([[0, 0], [1, 1], [2, 0]]));
+    });
+
+    it('sets the initial state', function() {
+      draw.extend(feature);
+      expect(draw.sketchCoords_).to.have.length(4);
+      expect(draw.sketchCoords_).to.eql([[0, 0], [1, 1], [2, 0], [2, 0]]);
+      expect(draw.finishCoordinate_).to.eql([2, 0]);
+    });
+
+    it('dispatches a drawstart event', function() {
+      var spy = sinon.spy();
+      goog.events.listen(draw, ol.interaction.DrawEventType.DRAWSTART, spy);
+      draw.extend(feature);
+      expect(spy.callCount).to.be(1);
+    });
+
+  });
 });
 
 goog.require('goog.dispose');
 goog.require('goog.events');
 goog.require('goog.events.BrowserEvent');
 goog.require('goog.style');
+goog.require('ol.Feature');
 goog.require('ol.Map');
 goog.require('ol.MapBrowserPointerEvent');
 goog.require('ol.View');