Add an observable input

[ar0ch]

Right now sever/rupture don't actually close the session (despite the naming), this PR adds an observable input (default ruptured). This allows the users to easily watch for the rupture event and do any clean up they need. For example, to automatically close the session server-side instead of waiting for the timeout:

observeEvent(input$ruptured, {
    session$close()
    })