Add data-id to pre block for code animations

[Arjen Wiersma]

Create the possibility to create code animations:

  <section data-background-image="img/bg/avocado.jpg" data-background-opacity="0.1" data-auto-animate>
    <pre data-id="code-animation"><code data-trim data-line-numbers class="clojure">
      (defn system [env]
      )
    </code></pre>
  </section>
  
  <section data-background-image="img/bg/avocado.jpg" data-background-opacity="0.1" data-auto-animate>
    <pre data-id="code-animation"><code data-trim data-line-numbers class="clojure">
      (defn system [env]
        (component/system-map
        :service-map (build-service-map env)
      )
    </code></pre>
  </section>

Currently data-id can not be created on pre blocks