Enable shiny shutdown hooks.

[useless5771]

Introduce a shutdown handler to which callback functions can be registered. The handler is then invoked if/when:

• shiny socket is closed
• startup of shiny app fails
• RCloud session is closed

[gordonwoodhull]

Awesome! It's great to have this functionality restored to shiny-overrides.R - this has been weighing on me for a long time.

And our users will definitely appreciate being able to do things when the window closes.

[gordonwoodhull]

Released in 0.5.4