add service worker config

package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "dev": "sapper dev",
     "build": "sapper build",
-    "start": "cross-env NODE_ENV=production node server.js",
+    "export": "sapper export",
+    "start": "cross-env NODE_ENV=production node .sapper/server.js",
     "cy:run": "cypress run",
     "cy:open": "cypress open",
     "test": "run-p --race dev cy:run"

webpack/service-worker.config.js

@@ -0,0 +1,17 @@
+const path = require('path');
+const config = require('sapper/webpack/config.js');
+const webpack = require('webpack');
+
+module.exports = {
+	entry: {
+		'service-worker': './app/service-worker.js'
+	},
+	output: {
+		path: path.resolve(`.sapper`),
+		filename: '[name].js',
+		chunkFilename: '[name].[id].[hash].js'
+	},
+	plugins: [
+		!config.dev && new webpack.optimize.ModuleConcatenationPlugin()
+	].filter(Boolean)
+};