Add purgecss to remove unused css, purgecss config and build config

purgecss.config.js

@@ -0,0 +1,15 @@
+module.exports = {
+  content: ['./public/**/*.html', './src/**/*.js'],
+  css: ['./src/styles/index.css'],
+  whitelist: ['body', 'html'],
+  extractors: [
+    {
+      extractor: class {
+        static extract(content) {
+          return content.match(/[A-z0-9-:\/]+/g)
+        }
+      },
+      extensions: ['html', 'js'],
+    },
+  ],
+}