For those "external links should open in new tabs" clients…

external.js

$('a').each(function() {
  if(this.host !== window.location.host) {
    $(this).attr('target', '_blank');
  }
});

external.vanilla.js

var a = document.querySelectorAll('a');

for(var i = 0; i < a.length; ++i) {
  if(a[i].host !== window.location.host) {
    a[i].target = '_blank';
  }
}