Make the source of the static assets configurable

- Easy to switch between different CDN
- Allow self-host static assets

_layouts/default.html

@@ -7,6 +7,8 @@ layout: compress
 
 {% include lang.html %}
 
+{% include assets-origin.html %}
+
 {% capture prefer_mode %}
   {% if site.theme_mode %}
     data-mode="{{ site.theme_mode }}"