berVi, instead of giving you a lecture I'll actually answer your question.
Its a very simple one line tweak.
Edit the file: include/parser.php
Find the handle_url_tag function
change the last line in this function from:
return '<a href="'.$full_url.'">'.$link.'</a>';
to
return '<a href="'.$full_url.'" target="_blank">'.$link.'</a>';
This may be frowned upon and not recommended but this is how you do it.