Support "Connection" property

[Aubron]

Per Auth0 Docs, it looks like there's a "Connection" property, that could allow one to throw straight to an expected social provider. It seems like it should be straightforward to map a configuration to provide this if specified.

Would be happy to contribute a PR if this sounds viable.

[dniel]

yeah,. should be easy enough to add, per application or as a global setting? If you add ass a PR I'll check it out and release it.