Option to keep window hidden on `did-finish-load`

[tofuness]

I think there should be an option to disable the automatic .show() on a browser window.

Use cases:

• We want the app to start in the tray and then open the main application window when the tray icon is clicked.

// Use case 1
app.on('ready', () => {
  const mainWindow = window.createWindow({
    width: 100,
    height: 100,
    show: false
  });
  mainWindow.showUrl(somePath);
  trayIcon.on('click', () => mainWindow.show());
});

• Application requires additional steps before it is ready to be shown.

// Use case 2
app.on('ready', () => {
  const mainWindow = window.createWindow({
    width: 100,
    height: 100,
    show: false
  });
  mainWindow.showUrl(somePath, () => {
    calculatePi();
    someAsyncFunction(() => {
        mainWindow.show();
    });
  });
});

[walleXD]

@tofuness You can use _loadURLWithArgs