cal: set featured_image in template

lumbunglib/templates/calendar.md

@@ -8,6 +8,9 @@ event_end: "{{ event.end }}"
 duration: "{{ event.duration }}"
 localized_begin: "{{ event.localized_begin }}"
 uid: "{{ event.uid }}"
+{% if event.featured_image %}
+featured_image: "{{ event.featured_image }}"
+{% endif %}
 {% if event.location %}
 location: "{{ event.location }}"
 {% endif %}