Extract HtmlConverter::convert() to an interface

Author: colinodell

CHANGELOG.md

@@ -7,6 +7,7 @@ Updates should follow the [Keep a CHANGELOG](http://keepachangelog.com/) princip
 ## [4.8.0] - 2018-09-18
 ### Added
  - Added support for email auto-linking
+ - Added a new interface (`HtmlConverterInterface`) for the main `HtmlConverter` class
  - Added additional test cases (#14)
 
 ### Changed

src/HtmlConverter.php

@@ -14,7 +14,7 @@
  *
  * @license http://www.opensource.org/licenses/mit-license.php MIT
  */
-class HtmlConverter
+class HtmlConverter implements HtmlConverterInterface
 {
     /**
      * @var Environment

src/HtmlConverterInterface.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace League\HTMLToMarkdown;
+
+/**
+ * Interface for an HTML-to-Markdown converter.
+ *
+ * @author Colin O'Dell <[email protected]>
+ *
+ * @link https://github.com/thephpleague/html-to-markdown/ Latest version on GitHub.
+ *
+ * @license http://www.opensource.org/licenses/mit-license.php MIT
+ */
+interface HtmlConverterInterface
+{
+    /**
+     * Convert the given $html to Markdown
+     *
+     * @param string $html
+     *
+     * @throws \InvalidArgumentException
+     *
+     * @return string The Markdown version of the html
+     */
+    public function convert($html);
+}