Define a minor mode for crux-reopen-as-root

Rather than changing the default find-file behavior, providing a minor
mode enables users to opt-in or toggle the behavior.

Fixes bbatsov#20.

crux.el

@@ -313,7 +313,14 @@ buffer is not visiting a file."
               (crux-file-owned-by-user-p buffer-file-name))
     (crux-find-alternate-file-as-root buffer-file-name)))
 
-(add-hook 'find-file-hook #'crux-reopen-as-root)
+;;;###autoload
+(define-minor-mode crux-reopen-as-root-mode
+  "Automatically reopen files as root if we can't write to them
+as the current user."
+  :global t
+  (if crux-reopen-as-root-mode
+      (add-hook 'find-file-hook #'crux-reopen-as-root)
+    (remove-hook 'find-file-hook #'crux-reopen-as-root)))
 
 (defun crux-insert-date ()
   "Insert a timestamp according to locale's date and time format."