Provide a way to unmount, or cleanup, a Gren application once it's started

This applies when mounting Gren applications within another app, and you want to remove. Typical example is a hybrid browser application, where you want to mount and unmount a Gren app when navigating to specific pages.

More info here. It applies to Elm, but it's the same case in Gren: https://gist.github.com/supermario/4c2615806c6c561a16edf5dd7208a759#js-modification-solution