Async abortable handlers

[rmja]

It would be really useful if the callback registered for a menu item could return a future, and if that future reflected the duration of the command such that the prompt is only shown after the future has completed. It should somehow be possible to abort the running command with e.g. Ctrl+C which should drop the future show the prompt.

[thejpster]

Async and blocking code isn't really compatible. Feel free to fork the crate as async-menu.