mirror weights

Please check google doc for detailed description and images

docs.google.com/document/d/1fUtvmVEgqrv3r0W7vBThXzXkjCswrDiy00vuhGoRboM/edit

Feature request Blender is missing a vital feature regarding weight painting objects deformed by armatures. That is to be able to mirror weights from one side of an axis over to the other side. For instance “copying” the weights on a characters left arm over to the right arm.