Add process method so extensions may process final AST without

copying implementation of `text`

Parsedown.php

@@ -36,7 +36,13 @@ class Parsedown
         $lines = explode("\n", $text);
 
         # iterate through lines to identify blocks
-        $markup = $this->lines($lines);
+        $Elements = $this->linesElements($lines);
+
+        # process elements
+        $Elements = $this->process($Elements);
+
+        # convert to markup
+        $markup = $this->elements($Elements);
 
         # trim line breaks
         $markup = trim($markup, "\n");
@@ -1709,6 +1715,11 @@ class Parsedown
         return $markup;
     }
 
+    protected function process(array $Elements)
+    {
+        return $Elements;
+    }
+
     # ~
 
     protected function li($lines)