Don’t autosave when the editor contains the focused element on focusout

This prevents autosave when things like the autocomplete menu are opened.

I intend to merge this in place of #15 because it's more general, but thanks so much to @lucas-clemente for the inspiration of using `event.relatedTarget`.

Fixes #3
Fixes atom/autocomplete#46
Fixes saschagehlich/autocomplete-plus#16
Closes #15