Add rel="noopener noreferrer" to all target="_blank" links

See https://mathiasbynens.github.io/rel-noopener/

src/Content/Text/HTML.php

@@ -943,7 +943,7 @@ class HTML
 	 */
 	public static function toLink($s)
 	{
-		$s = preg_replace("/(https?\:\/\/[a-zA-Z0-9\:\/\-\?\&\;\.\=\_\~\#\'\%\$\!\+]*)/", ' <a href="$1" target="_blank">$1</a>', $s);
+		$s = preg_replace("/(https?\:\/\/[a-zA-Z0-9\:\/\-\?\&\;\.\=\_\~\#\'\%\$\!\+]*)/", ' <a href="$1" target="_blank" rel="noopener noreferrer">$1</a>', $s);
 		$s = preg_replace("/\<(.*?)(src|href)=(.*?)\&amp\;(.*?)\>/ism", '<$1$2=$3&$4>', $s);
 		return $s;
 	}