Invoke a handler on client disconnect

I'm not sure if this is possible, but it would be cool to be able to call a `disconnected-handler` that takes an argument `user`. This way, if your handler creates any artifacts, the `disconnect-handler` can clean up after it.

I *think* [this](https://github.com/gliderlabs/sshfront/blob/master/handlers.go#L25) is part of the disconnect, but not sure if that's correct or not.