test: add test for nested gh codeblocks

test/cases/nested-gh-codeblocks.html

@@ -0,0 +1,8 @@
+<pre><code>1. some code idented 4 spaces
+
+    ```
+    var foobar = 'foo';
+    ```
+
+2. another line
+</code></pre>

test/cases/nested-gh-codeblocks.md

@@ -0,0 +1,9 @@
+```
+1. some code idented 4 spaces
+
+    ```
+    var foobar = 'foo';
+    ```
+
+2. another line
+```