[util] Make tools default to stdin if no text is provided

One less argument to type in typical testing workflow!
diff --git a/util/options.hh b/util/options.hh
index ec51ed3..bf9123b 100644
--- a/util/options.hh
+++ b/util/options.hh
@@ -239,7 +239,7 @@
     if (text && text_file)
       g_set_error (error,
 		   G_OPTION_ERROR, G_OPTION_ERROR_BAD_VALUE,
-		   "Only one of text and text-file must be set");
+		   "Only one of text and text-file can be set");
 
   };