Added vim modeline.

Added modeline with proper vim settings for urxvt perl lib. For some
reason, author of the urxvt choose 3 space indent for extension perl
files. I'd like to keep those for an ease of merging upstream changes.

tabbed

@@ -651,3 +651,5 @@ package urxvt::ext::tabbed::tab;
       die if $@;
    }
 }
+
+# vim: tabstop=3 softtabstop=3 shiftwidth=3 expandtab