Add extra_css block in base template

Inspired by {% block js %}

coin/templates/base.html

@@ -9,6 +9,7 @@
     <link rel="stylesheet" href="{% static "css/font-awesome.min.css"%}" />
     <link rel="stylesheet" href="{% static "css/local.css" %}" />
     <link rel="stylesheet" href="{% static "css/offcanvas.css" %}">
+    {% block extra_css %}{% endblock %}
     <script src="{% static "js/vendor/modernizr.js" %}"></script>
     <link rel="icon" type="image/png" href="{% static "img/coinitem.png" %}"/>
     <link rel="icon" type="image/x-icon" href="{% static "img/favicon.ico" %}" />