external-link-style.css

a.external {
  content: " \f08e";
  font: normal normal normal 14px/1 FontAwesome;
}

open-target-blank.php

<?php
/*
  Sets "external" class for external links and open them in a new tab/window
*/
function cs_bootstrap_external_link($text) {
  $wpUrl = get_bloginfo('url');
  $return = str_replace('href=', 'class="external" href=', $text);
  $return = str_replace('class="external" target="_blank" href="' . $wpUrl . '', 'href="' . $wpUrl . '', $return);
  $return = str_replace('class="external"  target="_blank" href="#', 'href="#', $return);
  return $return;
}
add_filter('the_content', 'cs_bootstrap_external_link');