[CORE][COMPOSER] Move plugins extlibs to composer (where appropriate)

vendor/michelf/php-markdown/Michelf/MarkdownInterface.inc.php

@@ -0,0 +1,9 @@
+<?php
+
+// Use this file if you cannot use class autoloading. It will include all the
+// files needed for the MarkdownInterface interface.
+//
+// Take a look at the PSR-0-compatible class autoloading implementation
+// in the Readme.php file if you want a simple autoloader setup.
+
+require_once dirname(__FILE__) . '/MarkdownInterface.php';