title: Symfony3: Enable aptoma twig-markdown extension
body: <p>How can I enable the <a href="https://github.com/aptoma/twig-markdown" rel="nofollow noreferrer">aptoma twig-markdown extension</a>? I have installed it using <code>composer</code> but when I use <code>{% markdown %}</code> in my twig file, I get an error message saying:</p>
  
  <blockquote>
    <p>Unexpected "markdown" tag (expecting closing tag for the "block" tag
    defined near line 8).</p>
  </blockquote>
  
  <p>I had a look in the symfony documentation but was not able to find the solution.</p>
  
  <p>Edit:</p>
  
  <p>I tried to add the following code to <code>services.yml</code> but got another error:</p>
  
  <pre><code>twig.markdown:
      class: Aptoma\Twig\Extension\MarkdownExtension
      arguments: []
      tags:
          - { name: twig.extension }
  </code></pre>
  
  <blockquote>
    <p>Type error: Argument 1 passed to
    Aptoma\Twig\Extension\MarkdownExtension::__construct() must be an
    instance of Aptoma\Twig\Extension\MarkdownEngineInterface, none given
    [...]</p>
  </blockquote>
  
  <p>Upon request my <code>.twig</code>-file:</p>
  
  <pre><code>{% extends 'XYZBundle::layout.html.twig' %}
  
  {% block title %}
      {{ parent() }} – Eintrag anzeigen
  {% endblock %}
  
  {% block platform_body %}
      &lt;ul&gt;
          &lt;li&gt;
              &lt;a href="{{ path('work_index') }}"&gt;Back to the list&lt;/a&gt;
          &lt;/li&gt;
          &lt;li&gt;
              &lt;a href="{{ path('work_edit', { 'id': work.id }) }}"&gt;Edit&lt;/a&gt;
          &lt;/li&gt;
      &lt;/ul&gt;
      &lt;h1&gt;{{ work.title }}&lt;/h1&gt;
  
      &lt;div class="work-content"&gt;
      {% markdown %}
      {{ work.content }}
      {% endmarkdown %}
      &lt;/div&gt;
  {% endblock %}
  </code></pre>