⬆️ 🔨 Upgrade PHP Markdown library.

The current version throws deprecated warning with PHP7.1 and PHPUnit.
Upgrade the Markdown library to the current PHP Markdown Lib 1.7.0.
Used composer to manage this library.

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';