Invoke a handler on client disconnect

[josegonzalez]

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 is part of the disconnect, but not sure if that's correct or not.

[progrium]

After a certain number of callbacks, I think we'd also want to change from args to config (env/file).