Use the new eleventy-plugin-bundle

_includes/layouts/post.njk

@@ -1,6 +1,11 @@
 ---
 layout: layouts/base.njk
 ---
+{%- css %}
+/* Only include the syntax highlighter CSS on blog posts */
+{% include "node_modules/prismjs/themes/prism-okaidia.css" %}
+{% include "public/css/prism-diff.css" %}
+{%- endcss %}
 <h1>{{ title }}</h1>
 
 <ul class="post-metadata">