format: support %Id conversion characters in format.js

This is needed for languages which translate numbers with an
alternative representation, such as Persian.

https://bugzilla.gnome.org/show_bug.cgi?id=673106

src/shell-util.c

@@ -868,3 +868,15 @@ shell_util_wifexited (int  status,
 
   return ret;
 }
+
+/**
+ * shell_format_int_alternative_output:
+ * @intval:
+ *
+ * Returns: (transfer full):
+ */
+gchar *
+shell_format_int_alternative_output (gint intval)
+{
+  return g_strdup_printf ("%Id", intval);
+}