Add the following piece of code just above the </head> tag:
<script src='http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js' type='text/javascript'></script>
<script type='text/javascript'> $(document).ready(function() {
$("a[href^='http://']").each(
function(){
if(this.href.indexOf(location.hostname) == -1) {
$(this).attr('target', '_blank');
}
}
);
$("a[href^='https://']").each( function(){
if(this.href.indexOf(location.hostname) == -1) {
$(this).attr('target', '_blank');
}
}
);
});
</script>
