support approval_prompt: force to get new refresh_token

[DTrejo]

It would be awesome if this were supported:

Important: When your application receives a refresh token, it is important to store that refresh token for future use. If your application loses the refresh token, it will have to re-prompt the user for consent before obtaining another refresh token. If you need to re-prompt the user for consent, include the approval_prompt parameter in the authorization code request, and set the value to force.
— https://developers.google.com/accounts/docs/OAuth2WebServer#offline

[DTrejo]

This can be achieved like so:

  // generate consent page url
  var url = oauth2Client.generateAuthUrl({
    access_type: 'offline', // will return a refresh token
    scope: 'https://www.googleapis.com/auth/calendar',
    approval_prompt: 'force'
  });