Refactoring for more consistency

* Min and max number of points configurable for lines and polygons
* Polygons from custom geometryFunction now have a sketch line
* The example shows how to use a custom geometryFunction

examples/draw-features.html

@@ -7,7 +7,9 @@ docs: >
   dropdown above to start drawing. To finish drawing, click the last
   point. To activate freehand drawing for lines and polygons, hold the `Shift`
   key. Square drawing is achieved by using Circle mode with a `geometryFunction`
-  that creates a 4-sided regular polygon instead of a circle.
+  that creates a 4-sided regular polygon instead of a circle. Box drawing uses a
+  custom `geometryFunction` that takes start and end point of a line with 2
+  points and creates a rectangular box.
 tags: "draw, edit, freehand, vector"
 ---
 <div class="row-fluid">
@@ -22,6 +24,7 @@ tags: "draw, edit, freehand, vector"
         <option value="Polygon">Polygon</option>
         <option value="Circle">Circle</option>
         <option value="Square">Square</option>
+        <option value="Box">Box</option>
       </select>
     </form>
   </div>