Add hook mechanism to quit command

[guang384]

Many times, I think it is not the best choice to re implement the exit command by inherit Quit and implement Quit.Command.

In most cases, I just want to perform some cleanup when issuing the exit command. I suggest that you can consider referring to the shutdown hook method of the runtime.

Runtime.getRuntime().addShutdownHook( () -> System.out.println("I'm shutdown hook...") );

Can you consider adding a "QuitCommandHook" mechanism instead of always rewriting the quit class to perform exit cleanup logic. The way envisaged is similar to:
Quit.addCommandHook( () -> System.out.println("I'm shutdown hook...") );

If possible, I would be happy to submit a PR on this issue.

[jvalkeal]

Have you thought about using any other means on your disposal to clean when application is going to exit? Bean destroy methods, application events from framework/boot etc?

I think jline has its own shutdown hook and spring has one as well in its core so it's a bit bad practice to add more of those as afaik hooks cannot be ordered.