Add webpack script/config to analyze bundle size.

Execute `npm run analyze` to open a size map of the production build in the browser.

config/webpack.bundle-analyzer.js

@@ -0,0 +1,11 @@
+const BundleAnalyzerPlugin = require("webpack-bundle-analyzer")
+  .BundleAnalyzerPlugin;
+
+const merge = require("webpack-merge");
+const prod = require("./webpack.prod.js");
+
+const config = merge(prod, {
+  plugins: [new BundleAnalyzerPlugin()]
+});
+
+module.exports = config;