Optimize generated CSS by removing unused selectors (#2008)

Hugo generates stats about the HTML elements, IDs and classes that can be found in the website,
and we post-process the rendered CSS with postcss-purgecss that uses those stats to remove unused selectors.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

postcss.config.js

@@ -0,0 +1,15 @@
+// Remove unused CSS selectors.
+const purgecss = require('@fullhuman/postcss-purgecss')({
+    // Use stats generated by Hugo.
+    content: [ './hugo_stats.json' ],
+    defaultExtractor: (content) => {
+        let els = JSON.parse(content).htmlElements;
+        return els.tags.concat(els.classes, els.ids);
+    }
+});
+
+module.exports = {
+     plugins: [
+         ...(process.env.HUGO_ENVIRONMENT === 'production' ? [ purgecss ] : [])
+     ]
+ };