Add new prelude custom variable for user-init-file

This way the prelude-find-user-init-file can actually take you to a
useful file (instead of to init.el)

core/prelude-core.el

@@ -553,7 +553,7 @@ This follows freedesktop standards, should work in X servers."
 (defun prelude-find-user-init-file ()
   "Edit the `user-init-file', in another window."
   (interactive)
-  (find-file-other-window user-init-file))
+  (find-file-other-window prelude-user-init-file))
 
 (defun prelude-find-shell-init-file ()
   "Edit the shell init file in another window."

core/prelude-custom.el

@@ -64,6 +64,15 @@ Will only occur if prelude-whitespace is also enabled."
   :type 'boolean
   :group 'prelude)
 
+(defcustom prelude-user-init-file (concat user-emacs-directory
+                                          "personal/")
+  "Path to your personal customization file.
+
+Since Prelude recommends you only put personal customizations in
+the personal folder."
+  :type 'string
+  :group 'prelude)
+
 (provide 'prelude-custom)
 
 ;;; prelude-custom.el ends here