JS SignalR - HubConnection - Ability to unregister callbacks for connection states.

I was writing an internal plugin when I realized that there is no way to remove a callback added via `onreconnected`, save for creating a whole new hub connection. It could be useful to be able to add and remove temporary callbacks. For example inside a certain page, do some extra work if the connection is reestablished, unregister it before leaving the page.

The only workaround for now would be to have a single callback that would call from an array that I can modify, though it would be nicer if signalR provided a method.