Allow disconnecting a session via custom handler

[josegonzalez]

It would be great to have some clock process every N seconds that could invoke a disconnection-handler. The handler would exit 0 to disconnect, and exit 1 to stay alive.

This would be useful for something like allowing for SSH session TTLs, or revoking SSH access at run-time through a call to an external api/service.

[progrium]

Do you think that's a sufficiently advanced case that you could just use gliderlabs/ssh instead? (Which this should eventually be rewritten with)