Define a minor mode for crux-reopen-as-root (#28)

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

Fixes #20.

README.md

@@ -114,6 +114,14 @@ current line, rather than the whole line, in the absence of a region:
 (crux-with-region-or-point-to-eol kill-ring-save)
 ```
 
+## Minor modes
+
+#### `(crux-reopen-as-root-mode)` ####
+
+Crux provides a `crux-reopen-as-root` command for reopening a file as
+root. This global minor mode changes `find-file` so all root files are
+automatically opened as root.
+
 ## License
 
 Copyright © 2015 Bozhidar Batsov and [contributors][].

crux.el

@@ -327,7 +327,14 @@ 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."