Allow specifying a WS closing code to TemplatedApp.close()

Hello,

As per documentation, the TemplatedApp.close() method "closes all sockets including listen sockets". For WS servers this means that all WS connections are closed without a closing code (I'm guessing the underlying TCP socket is the one that is actually closed) and the reported code is 1006.

It would be useful to have a way for specifying the WS closing code to send to the clients. This way, applications can identify and handle on the client side the situation of a server being closed.