Add mermaid support (#857)

Closes #825


Co-authored-by: Matt Wang <matt@matthewwang.me>

_sass/code.scss

@@ -139,4 +139,11 @@ figure.highlight {
   }
 }
 
+// Mermaid diagram code blocks should be left unstyled.
+code.language-mermaid {
+  padding: 0;
+  background-color: inherit;
+  border: 0;
+}
+
 // {% endraw %}