Fix loading of index.html

#40
https://electron.atom.io/docs/tutorial/quick-start/

src/index.js

@@ -1,5 +1,7 @@
 const electron = require('electron'); // eslint-disable-line
 const isDev = require('electron-is-dev');
+const path = require('path');
+const url = require('url');
 
 const menu = require('./menu');
 
@@ -18,7 +20,11 @@ function createWindow() {
   mainWindow = new BrowserWindow({
     darkTheme: true,
   });
-  mainWindow.loadURL(`file://${__dirname}/index.html`);
+  mainWindow.loadURL(url.format({
+    pathname: path.join(__dirname, 'index.html'),
+    protocol: 'file:',
+    slashes: true,
+  }));
 
   mainWindow.on('closed', () => {
     // Dereference the window object, usually you would store windows