v4.0.0

⚠ BREAKING CHANGES

• This commit creates multiple breaking changes. The getToken()
  method previously returned Promise<string>, where the string was the
  access_token returned from the response. However, the oauth2 endpoint could
  return a variety of other fields, such as an id_token in special cases.

const token = await getToken();
// old response: 'some.access.token'
// new response: { access_token: 'some.access.token'}

To further support this change, the GoogleToken class no longer exposes
a token variable. It now exposes rawToken, accessToken, and idToken
fields which can be used to access the relevant values returned in the
response.

Bug Fixes

• expose all fields from response (#218) (d463370)