Add support for Plausible analytics

Introduce a new configuration option, PLAUSIBLE_DOMAIN. If set,
include the Plausible (https://plausible.io) analytics script for that
domain.

Fixes #317.

templates/base.html

@@ -100,6 +100,8 @@
   {% include "partial/ga.html" %}
   {% include "partial/ggst.html" %}
 
+  {% include "partial/plausible.html" %}
+
   {% if not PLUGINS or 'seo' not in PLUGINS %}
     {% if REL_CANONICAL %}
       {% if page %}

templates/partial/plausible.html

@@ -0,0 +1,3 @@
+{% if PLAUSIBLE_DOMAIN %}
+<script defer data-domain="{{ PLAUSIBLE_DOMAIN }}" src="https://plausible.io/js/plausible.js"></script>
+{% endif %}