Refactor into namespaces for PSR-4

tests/data/code_block.md

@@ -0,0 +1,17 @@
+    <?php
+
+    $message = 'Hello World!';
+    echo $message;
+
+---
+
+    > not a quote
+    - not a list item
+    [not a reference]: http://foo.com
+
+---
+
+    foo
+
+
+    bar