Add markdown format support (#25)

Co-authored-by: Dawid Dziurla <dawidd0811@gmail.com>

node_modules/showdown/test/cases/blockquote-nested-markdown.html

@@ -0,0 +1,10 @@
+<blockquote>
+  <h2 id="thisisaheader">This is a header.</h2>
+  <ol>
+    <li>This is the first list item.</li>
+    <li>This is the second list item.</li>
+  </ol>
+  <p>Here's some example code:</p>
+    <pre><code>return shell_exec("echo $input | $markdown_script");
+    </code></pre>
+</blockquote>