title: Sort on user defined properties
body: <p>I'm using hugo v0.15
  I'm trying to sort pages on custom property.</p>
  
  <p>I define my subpages as following:</p>
  
  <pre><code>+++
  title= "bla bla bla"
  parent = "parent"
  index = 0 # each page is assigned a unique index
  +++
  </code></pre>
  
  <p>And in my parent template as following:</p>
  
  <pre><code>{{ range sort .Site.Pages ".Params.index" }}
      &lt;a href="{{.RelPermalink}}"&gt;
            {{.Title}} {{.Params.index}}
      &lt;/a&gt;
  {{end}}
  </code></pre>
  
  <p>this doesn't fails at compilation, but the list is rendered empty.
  what am i missing?</p>