Improve prelude-open-with

Now it uses `xdg-open` where it makes sense. It also accepts a prefix
argument that forces manual selection of a command to run.

core/prelude-core.el

@@ -35,14 +35,17 @@
 (require 'thingatpt)
 (require 'dash)
 
-(defun prelude-open-with ()
-  "Open visited file in external program."
-  (interactive)
+(defun prelude-open-with (arg)
+  "Open visited file in default external program.
+
+With a prefix ARG always prompt for command to use."
+  (interactive "P")
   (when buffer-file-name
     (shell-command (concat
-                    (if (eq system-type 'darwin)
-                        "open"
-                      (read-shell-command "Open current file with: "))
+                    (cond
+                     ((and (not arg) (eq system-type 'darwin)) "open")
+                     ((and (not arg) (member system-type '(gnu gnu/linux gnu/kfreebsd))) "xdg-open")
+                     (t (read-shell-command "Open current file with: ")))
                     " "
                     buffer-file-name))))