_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?

config/asset-template-plugin.js

@@ -0,0 +1,47 @@
+const path = require('path');
+const fs = require('fs');
+const ejs = require('ejs');
+const AssetsPlugin = require('assets-webpack-plugin');
+
+module.exports = class AssetTemplatePlugin extends AssetsPlugin {
+  constructor(options) {
+    options = options || {};
+    if (!options.template) throw Error('AssetTemplatePlugin: template option is required.');
+    super({
+      useCompilerPath: true,
+      filename: options.filename,
+      processOutput: files => this._processOutput(files)
+    });
+    this._template = path.resolve(process.cwd(), options.template);
+    const ignore = options.ignore || /(manifest\.json|\.DS_Store)$/;
+    this._ignore = typeof ignore === 'function' ? ({ test: ignore }) : ignore;
+  }
+
+  _processOutput(files) {
+    const mapping = {
+      all: [],
+      byType: {},
+      entries: {}
+    };
+    for (const entryName in files) {
+      // non-entry-point-derived assets are collected under an empty string key
+      // since that's a bit awkward, we'll call them "assets"
+      const name = entryName === '' ? 'assets' : entryName;
+      const listing = files[entryName];
+      const entry = mapping.entries[name] = {
+        all: [],
+        byType: {}
+      };
+      for (let type in listing) {
+        const list = [].concat(listing[type]).filter(file => !this._ignore.test(file));
+        if (!list.length) continue;
+        mapping.all = mapping.all.concat(list);
+        mapping.byType[type] = (mapping.byType[type] || []).concat(list);
+        entry.all = entry.all.concat(list);
+        entry.byType[type] = (entry.byType[type] || []).concat(list);
+      }
+    }
+    mapping.files = mapping.all;
+    return ejs.render(fs.readFileSync(this._template, 'utf8'), mapping);
+  }
+};