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);
```

src/ol/interaction/drawinteraction.js

@@ -714,6 +714,33 @@ ol.interaction.Draw.prototype.abortDrawing_ = function() {
 };
 
 
+/**
+ * Extend an existing geometry by adding additional points. This only works
+ * on features with `LineString` geometries, where the interaction will
+ * extend lines by adding points to the end of the coordinates array.
+ * @param {!ol.Feature} feature Feature to be extended.
+ * @api
+ */
+ol.interaction.Draw.prototype.extend = function(feature) {
+  var geometry = feature.getGeometry();
+  goog.asserts.assert(this.mode_ == ol.interaction.DrawMode.LINE_STRING,
+      'interaction mode must be "line"');
+  goog.asserts.assert(goog.isDefAndNotNull(geometry),
+      'feature must have a geometry');
+  goog.asserts.assert(geometry.getType() == ol.geom.GeometryType.LINE_STRING,
+      'feature geometry must be a line string');
+  var lineString = /** @type {ol.geom.LineString} */ (geometry);
+  this.sketchFeature_ = feature;
+  this.sketchCoords_ = lineString.getCoordinates();
+  var last = this.sketchCoords_[this.sketchCoords_.length - 1];
+  this.finishCoordinate_ = last.slice();
+  this.sketchCoords_.push(last.slice());
+  this.updateSketchFeatures_();
+  this.dispatchEvent(new ol.interaction.DrawEvent(
+      ol.interaction.DrawEventType.DRAWSTART, this.sketchFeature_));
+};
+
+
 /**
  * @inheritDoc
  */