Allow over-riding redirectURL in authenticate() arguments

[ajmas]

Right now there does not seem to be a way to over-ride the redirectUrl via the authenticate() call, so it would be useful to provide an optional third options argument to do this:

authenticate(provider, userData, options = {}) {
   var providerConfig = Object.assign({}, this.options.providers[provider], options);
   ...

The use case we have is that we have one screen for linking the account and another for actually using the account to login.