_headers & _redirects generation (#240)

* Generate `_headers` and `_redirects` by passing assets through ejs templates.

* PR feedback

* Excluding service worker stuff from prerender

* Build SW in dev

* Let's give this a try

* lol

* Is this how it works?

webpack.config.js

@@ -14,6 +14,7 @@ const BundleAnalyzerPlugin = require('webpack-bundle-analyzer').BundleAnalyzerPl
 const WorkerPlugin = require('worker-plugin');
 const AutoSWPlugin = require('./config/auto-sw-plugin');
 const CrittersPlugin = require('critters-webpack-plugin');
+const AssetTemplatePlugin = require('./config/asset-template-plugin');
 
 function readJson (filename) {
   return JSON.parse(fs.readFileSync(filename));
@@ -238,8 +239,11 @@ module.exports = function (_, env) {
         compile: true
       }),
 
-      new AutoSWPlugin({
-        version: VERSION
+      new AutoSWPlugin({ version: VERSION }),
+
+      isProd && new AssetTemplatePlugin({
+        template: path.join(__dirname, '_headers.ejs'),
+        filename: '_headers',
       }),
 
       new ScriptExtHtmlPlugin({