title: Use variable and filter within tag in nunjucks + eleventy
body: <p>I am a nunjucks newbie - trying it out along with eleventy.</p>
  
  <p><strong>Short version:</strong> can I use variables and filters within tags in nunjucks? For example:</p>
  
  <p>{% set myVar = {{ title | lower }} %}</p>
  
  <p>(assume the {{ title }} variable is set correctly)</p>
  
  <p><strong>Longer version / my specific use case:</strong></p>
  
  <p>I am trying to create a layout file called section.njk and use it for several pages (basically the section frontpages of each section of my site - similar idea to a section.html layout file in Hugo) I have a data file for each section, containing the menu for that section.</p>
  
  <p>Given the following files:</p>
  
  <ul>
  <li>guides.json </li>
  <li>index.md with frontmatter including title: "Guides"</li>
  </ul>
  
  <p>This does not work:</p>
  
  <pre><code>{% for item in {{ title | lower }} %}
  &lt;a href="{{ item.url | url }}"&gt;&lt;div&gt;{{ item.title }}&lt;/div&gt;&lt;/a&gt;
  {% endfor %}
  </code></pre>
  
  <p>This works:</p>
  
  <pre><code>{% for item in guides %}
  &lt;a href="{{ item.url | url }}"&gt;&lt;div&gt;{{ item.title }}&lt;/div&gt;&lt;/a&gt;
  {% endfor %}
  </code></pre>
  
  <p>So I'm wondering if the problem is using variables within tags in Nunjucks, and if so, if there's a way round it?</p>
  
  <p>Entire project source is here: <a href="https://github.com/StarfallProjects/tech-writer-toolkit/tree/11ty" rel="nofollow noreferrer">https://github.com/StarfallProjects/tech-writer-toolkit/tree/11ty</a>  (the 11ty stuff is in src)</p>
  
  <p>Thanks for any help.</p>