Make the source of the static assets configurable

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

_includes/jsdelivr-combine.html

@@ -0,0 +1,32 @@
+{% assign urls = include.urls | split: ',' %}
+
+{% assign combined_urls = nil %}
+
+{% assign domain = 'https://cdn.jsdelivr.net/' %}
+
+{% for url in urls %}
+
+  {% if url contains domain %}
+    {% assign url_snippet = url | slice: domain.size, url.size %}
+
+    {% if combined_urls %}
+      {% assign combined_urls = combined_urls | append: ',' | append: url_snippet %}
+    {% else %}
+      {% assign combined_urls = domain | append: 'combine/' | append: url_snippet %}
+    {% endif %}
+
+  {% elsif url contains '//' %}
+
+    <script src="{{ url }}"></script>
+
+  {% else %}
+
+    <script src="{{ url | relative_url }}"></script>
+
+  {% endif %}
+
+{% endfor %}
+
+{% if combined_urls %}
+  <script src="{{ combined_urls }}"></script>
+{% endif %}