Added h5 and h6

Author: JoshPiper

src/Converter.php

@@ -36,6 +36,8 @@ public function __construct(string $text = null){
 		'h2' => '##',
 		'h3' => '###',
 		'h4' => '####',
+		'h5' => '#####',
+		'h6' => '######',
 	];
 
 	/** List of bbTags with attributes and their replacements.