Undo persistence

[alexfertel]

I really miss the undofile configuration option from vim. I would love to have this builtin, or as an extension. I'm happy to implement this feature if the team is down.

5. Undo persistence

When unloading a buffer Vim normally destroys the tree of undos created for
that buffer. By setting the 'undofile' option, Vim will automatically save
your undo history when you write a file and restore undo history when you edit
the file again.