function applyExternalLink() {
  var oAnchors = document.getElementsByTagName( 'a' );
  for( var i=0; i<oAnchors.length; i++ )
    if ( oAnchors[ i ].className.indexOf( 'external_link' ) != -1 )
      oAnchors[ i ].target = '_blank';
}

addListener( window, 'load', applyExternalLink );