add service worker config

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)
+};