load_template() plugin hook

Closes #1042

datasette/templates/base.html

@@ -79,6 +79,10 @@ document.body.addEventListener('click', (ev) => {
     <script>{{ body_script }}</script>
 {% endfor %}
 
-{% if select_templates %}<!-- Templates considered: {{ select_templates|join(", ") }} -->{% endif %}
+{% if templates_considered %}
+<!-- Templates considered:
+{% for template in templates_considered %}- {{ template.name }}{% if template.used %} (used{% if template.from_plugin %}, from plugin{% endif %}){% endif %}
+{% endfor %}-->
+{% endif %}
 </body>
 </html>