Support Twig engine in 7.x

http://makina-corpus.com/blog/metier/2015/howto-using-twig-in-drupal-7

It would be nice if we could keep some parity between the 8.x and 7.x versions. Obviously we cannot remove *.tpl.php files in 7.x, but perhaps it would be nice to offer a solution for those whom wish to use Twig in D7?

From reading the above article, we can alter the system info from Bootstrap itself using hook_system_info_alter():

/**
 * Implements hook_system_info_alter().
 */
function bootstrap_system_info_alter(&$info, $file, $type) {
  // @todo Add support for grand-child themes.
  if ($type == 'theme' && module_exists('tfd7') && ($info['name'] === 'bootstrap' || $info['base theme'] === 'bootstrap')) {
    $info['engine'] = 'twig';
  }
}

I'm not entirely sure that this is the only thing needed and we'll likely need to provide sub-theme support as well. Thoughts?