Add :emacs undo module

Moves undo-fu/undo-tree out of core and allows uses to choose one or the
other.

Relevant to #2339

init.example.el

@@ -66,6 +66,7 @@
        dired             ; making dired pretty [functional]
        electric          ; smarter, keyword-based electric-indent
        ;;ibuffer         ; interactive buffer management
+       undo              ; persistent, smarter undo for your inevitable mistakes
        vc                ; version-control and Emacs, sitting in a tree
 
        :term