[Feature Request] Add a on close command

I usually run tmux on my servers so when I log back in I have my session still in place. However, when running in the background and the app shuts down there is no message sent to detatch the session. It would be nice if just like the command to run when connecting we had a similar option for a command to run when the connection is closing.

Find the existing command option that runs when a connection is established, then trace the connection-closing lifecycle where a corresponding option could be handled. Done means users can configure a command that runs when the connection closes, including for background sessions such as tmux.

Written by the indexing model from the issue text.