Support #hashtag per CommonMark and GFM specs

Parsedown.php

@@ -519,6 +519,11 @@ class Parsedown
             }
 
             $text = trim($Line['text'], '#');
+
+            if (!isset($text[0]) or $text[0] !== ' ') {
+                return;
+            }
+
             $text = trim($text, ' ');
 
             $Block = array(

test/data/atx_heading.html

@@ -7,5 +7,7 @@
 <p>####### not a heading</p>
 <h1>closed h1</h1>
 <p>#</p>
+<p>##</p>
 <h1># of levels</h1>
-<h1># of levels #</h1>
+<h1># of levels #</h1>
+<p>#hashtag</p>

test/data/atx_heading.md

@@ -16,6 +16,10 @@
 
 #
 
+##
+
 # # of levels
 
-# # of levels # #
+# # of levels # #
+
+#hashtag