perf: tree shaking Bootstrap CSS (#1736)

purgecss.config.js

@@ -0,0 +1,23 @@
+const fs = require('fs');
+const DIST_PATH = '_sass/dist';
+
+fs.rm(DIST_PATH, { recursive: true, force: true }, (err) => {
+  if (err) {
+    throw err;
+  }
+
+  fs.mkdirSync(DIST_PATH);
+});
+
+module.exports = {
+  content: ['_includes/**/*.html', '_layouts/**/*.html', '_javascript/**/*.js'],
+  css: ['node_modules/bootstrap/dist/css/bootstrap.min.css'],
+  keyframes: true,
+  variables: true,
+  output: `${DIST_PATH}/bootstrap.css`,
+  // The `safelist` should be changed appropriately for future development
+  safelist: {
+    standard: [/^collaps/, /^w-/, 'shadow', 'border', 'kbd'],
+    greedy: [/^col-/, /tooltip/]
+  }
+};