feat(snapping): snap to source element

This ensures we snap to a source element if one is provided, i.e. during
shape.append.

lib/features/snapping/BpmnSnapping.js

@@ -193,6 +193,13 @@ BpmnSnapping.prototype.initSnap = function(event) {
         y: docking.y - event.y
       });
     });
+
+  }
+
+  var source = context.source;
+
+  if (source) {
+    snapContext.addDefaultSnap('mid', mid(source));
   }
 };
 
@@ -231,4 +238,5 @@ BpmnSnapping.prototype.addTargetSnaps = function(snapPoints, shape, target) {
       snapPoints.add(c.id + '-docking', docking.original || docking);
     }
   });
+
 };