title: Loop through a directory and its sub-directories files in liquid
body: <p>I am a newbie to liquid templating language, regarding the website am working on it's powered by jekyll, we publish posts and sometimes series of posts so we make a sub directory for series posts in <code>_posts</code> directory. Can I loop through all files in the directory <code>_posts</code> and its sub directories given that I don't know their names or number ? <br>
  My file tree :<br>
  <code>root
     |
     _posts/
        |
        post1.md
        post2.md
        series_name/
           |
           post3.md</code></p>