Support for onbeforeunload browser alert

The post editing screen in WordPress includes an `onbeforeunload` event handler which triggers the browser's "Do you want to leave this site? Changes you made may not be saved." alert box if you've entered a value into the post title or post content area and then attempt to navigate away from the page without saving.

It would be great if Fieldmanager supported this functionality for fields within custom meta boxes. That way, if a user enters a bunch of information into custom field inputs (not including the post title and post content), and the attempts to navigate away from the page, they'll see the same alert.

I'm not sure if should be an opt-in parameter when registering fields, or whether it should default to this behaviour.

Thoughts?