This task becomes relatively easy with the help of our friend jQuery. The code below is almost too simple!
$('a').each(function() {
url = $(this).attr('href');
//detect external link
if ( (url.indexOf('http') >= 0 || url.indexOf('https') >= 0) && (url.indexOf('yoursitedomain.com') < 0))
$(this).attr('target', '_blank');
});