⬆️ 🔨 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.php

@@ -0,0 +1,38 @@
+<?php
+/**
+ * Markdown  -  A text-to-HTML conversion tool for web writers
+ *
+ * @package   php-markdown
+ * @author    Michel Fortin <michel.fortin@michelf.com>
+ * @copyright 2004-2016 Michel Fortin <https://michelf.com/projects/php-markdown/>
+ * @copyright (Original Markdown) 2004-2006 John Gruber <https://daringfireball.net/projects/markdown/>
+ */
+
+namespace Michelf;
+
+/**
+ * Markdown Parser Interface 
+ */
+interface MarkdownInterface {
+	/**
+	 * Initialize the parser and return the result of its transform method.
+	 * This will work fine for derived classes too.
+	 *
+	 * @api
+	 *
+	 * @param  string $text
+	 * @return string
+	 */
+	public static function defaultTransform($text);
+
+	/**
+	 * Main function. Performs some preprocessing on the input text
+	 * and pass it through the document gamut.
+	 *
+	 * @api
+	 *
+	 * @param  string $text
+	 * @return string
+	 */
+	public function transform($text);
+}