Make the source of the static assets configurable

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

_includes/mermaid.html

@@ -2,7 +2,7 @@
   mermaid-js loader
 -->
 
-<script src="https://cdn.jsdelivr.net/npm/mermaid@8/dist/mermaid.min.js"></script>
+<script src="{{ site.data.assets[origin].mermaid.js }}"></script>
 
 <script>
   $(function() {