There's no easy way to do it, and like @alex has noted you'll need to use JavaScript. His answer is the best solution but in order to optimize it, you might want to filter only to the post-content links.
<script>
var links = document.querySelectorAll( '.post-content a' );
for (var i = 0, length = links.length; i < length; i++) {
if (links[i].hostname != window.location.hostname) {
links[i].target = '_blank';
}
}
</script>
The code is compatible with IE8+ and you can add it to the bottom of your page. Note that you'll need to change the ".post-content a" to the class that you're using for your posts.
As seen here: http://blog.hubii.com/target-_blank-for-links-on-ghost/