feather/_includes/category.html

<div class="pv4">
    <h2 class="mt0 mb4 f6 ttu tracked normal">{{ include.category | replace: '-', ' '}}</h2>
    <ul class="icon-grid mv0 pl0 list">
      {% capture category_path %}icons/{{ include.category }}{% endcapture %}
      {% for icon in site.static_files %}
        {% if icon.path contains category_path  %}
          {% include icon.html icon=icon %}
        {% endif %}
      {% endfor %}
    </ul>
  </div>