title: Jinja2 {% include file %} outside of search path doesn't work
body: <p>This is an elementary issue which is probably related to Jinja2 PrefixLoader or ChoiceLoader. </p>
  
  <p>On Python 3.6 we load with this command </p>
  
  <pre><code>jinja2.FileSystemLoader( searchpath= "\\template_folder\\")
  </code></pre>
  
  <p>On Windows 7, our file structure is as follows.</p>
  
  <pre>
  - folder_bbb
      * subfile.txt
  - template_folder
       * template_file
       - folder_aaa
           * subfile.txt
  </pre>
  
  <p>From the template_file we are successful with this command </p>
  
  <pre><code>{% include "folder_aaa/subfile.txt" %} 
  </code></pre>
  
  <p>Now we wish to move the file one level up, and write</p>
  
  <pre><code>{% include "../folder_bbb/subfile.txt" %}
  </code></pre>
  
  <p>but that doesn't work, complaining file not found.</p>
  
  <p>What is the correct way to write? Thanks.</p>