site/_src/_includes/menu-footer.html

<nav class="menu menu--footer">
    <div class="row">
        {% for link in site.data.menus.footer %}

            {% assign active = nil %}
            {% if page.url contains link.url %}
                {% assign active = 'active' %}
            {% endif %}

            <a class="menu__link" class="{{ active }}" href="{{ link.url }}">{{ link.title }}</a>

        {% endfor %}
    </div>
</nav>