implement GFM strikethrough

Parsedown.php

@@ -766,6 +766,11 @@ class Parsedown
 
 		# ~
 
+		if (strpos($text, '~~') !== FALSE)
+		{
+			$text = preg_replace('/~~(?=\S)(.+?)(?<=\S)~~/s', '<del>$1</del>', $text);
+		}
+
 		if (strpos($text, '_') !== FALSE)
 		{
 			$text = preg_replace('/__(?=\S)(.+?)(?<=\S)__(?!_)/s', '<strong>$1</strong>', $text);

tests/data/strikethrough.html

@@ -0,0 +1,3 @@
+<p><del>strikethrough</del></p>
+<p>in the <del>middle</del> of a sentence</p>
+<p>in the middle of a w<del>or</del>d</p>

tests/data/strikethrough.md

@@ -0,0 +1,5 @@
+~~strikethrough~~
+
+in the ~~middle~~ of a sentence
+
+in the middle of a w~~or~~d