feature: add 'addScopes' to useGoogleLogin hook to support incremental scopes

[flandrade]

Google Sign-In allows requesting additional permissions as described in the official documentation. In this PR, I follow the proposed approach in #348:

The useGoogleLogin hook exposes the new addScopes method. This hook also supports two new callbacks, onAddScopesSuccess and onAddScopesFailure, to handle the method response:

import { useGoogleLogin } from 'react-google-login'

const { addScopes } = useGoogleLogin({
  clientId,
  onAddScopesSuccess,
  onAddScopesSuccess
})

NOTE: Since there are two accessType to authorize scopes: online and offline. The addScopes method will use user.grant(options) or auth.grantOfflineAccess(options). This method would be only executed if the user is logged in.

This PR is for #348 and #371.