Add support for both client_id and clientId

[billyyyyy3320]

Currently, the example in README doesn't work fine.

It only accepts client_id , but the example is using clientId:

      googleSignInParams: {
        clientId: 'YOUR_APP_CLIENT_ID.apps.googleusercontent.com'
      }

I tested it and found that both client_id and clientId could be accepted by google api.

[phanan]

LGTM

[phanan]

Thanks!