Add prod and dev configs, dev-server

- Broke our webpack.config file into 3 files
- webpack.common.js, webpack.dev.js, and webpack.prod.js
- installed webpack-merge to share the common functionality
- updated our package.json to use the new config files
- installed webpack-dev-server and added it to start script in package.json

webpack.prod.js

@@ -0,0 +1,11 @@
+const path = require("path");
+const common = require("./webpack.common");
+const merge = require("webpack-merge");
+
+module.exports = merge(common, {
+  mode: "production",
+  output: {
+    filename: "main.[contentHash].js",
+    path: path.resolve(__dirname, "dist")
+  }
+});