* As explained in: electron/electron#8862 (comment)
    an issue with .quit() exists with a "Cannot find module '../screen'" issue,
    while using the .exit() alternative avoids the issue

* Validated on Windows with the same logic as #744 where the issue recently appeared

…ers (nativefier#744)

[Squirrel](https://github.com/Squirrel/Squirrel.Windows) is *"an installation and update
framework for Windows desktop apps "*.

This PR adds `electron-squirrel-startup`, allowing to package nativefier applications 
into squirrel-based setup installers. Squirrel require this entrypoint to perform
desktop and startup menu creations, without showing the UI on setup launches.

- References: https://github.com/mongodb-js/electron-squirrel-startup

- Resolves `electron-winstaller` and `electron-installer-windows` support of desktop / startup menu shortcuts for nativefier packaged applications.

- The `electron-squirrel-startup` entrypoint has no effect on both Linux and Darwin, only on Windows

- Supporting it directly inside `nativefier` avoids having to "hack" around the existing `main.js`
  and including dependencies from `electron-squirrel-startup` in an intermediate package
  to be included in a third layer for the final installer executable

- The following script based on both `nativefier` and `electron-winstaller` templates
   represents a portable proof of concept for this merge request :

```js
var nativefier = require('nativefier').default;
var electronInstaller = require('electron-winstaller');

var options = {
  name: 'Web WhatsApp',
  targetUrl: 'http://web.whatsapp.com',
  platform: 'windows',
  arch: 'x64',
  version: '0.36.4',
  out: '.',
  overwrite: false,
  asar: false,
  counter: false,
  bounce: false,
  width: 1280,
  height: 800,
  showMenuBar: false,
  fastQuit: false,
  userAgent: 'Mozilla ...',
  ignoreCertificate: false,
  ignoreGpuBlacklist: false,
  enableEs3Apis: false,
  insecure: false,
  honest: false,
  zoom: 1.0,
  singleInstance: false,
  fileDownloadOptions: {
    saveAs: true
  },
  processEnvs: {
    GOOGLE_API_KEY: '<your-google-api-key>'
  }
};

nativefier(options, function(error, appPath) {
  if (error) {
    console.error(error);
    return;
  }
  console.log('App has been nativefied to', appPath);

  resultPromise = electronInstaller.createWindowsInstaller({
    appDirectory: 'Web WhatsApp-win32-x64',
    outputDirectory: './',
    authors: 'Web WhatsApp',
    exe: 'Web WhatsApp.exe'
  });

  resultPromise.then(() => console.log('It worked!'), e => console.log(`No dice: ${e.message}`));
});
```

…#784)

 * As explained in: electron/electron#8862 (comment)
    an issue with .quit() exists with a "Cannot find module '../screen'" issue,
    while using the .exit() alternative avoids the issue

* Validated on Windows with the same logic as nativefier#744 where the issue recently appeared