Allow `onlogin` callback for `widget.show`. Fix #32

This PR adds an optional 3rd parameter to `widget.show` | `widget.signin` on **popup** mode, Which, along with `callbackOnLocationHash` turns to be mandatory.

As explained in #32, the callback function should be able to receive all parameters delegated from `auth0-js` callback for `.login` as shown here: https://github.com/auth0/auth0.js#login