Add undo module

Was previously included in core-editor, now can be disabled, as of
hlissner/doom-emacs@3e340ab.

init.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