Add ClassLink Provider

[ryan-berger]

The organization I work for heavily uses goth, so we wanted to contribute back with a ClassLink provider that we will be using in future releases of our product.

It is available on https://github.com/SenorWoolyLLC/goth/tree/classlink

The provider is nearly ready to go (still some documenting/testing to do) but I wanted to open this issue as a heads-up so that no one else implements it (unless you can beat me to it of course).

In order to get test credentials to test our provider out we require ClassLink approval, so we're still waiting for that and then we'll be able to open the PR as soon as we can verify it is functioning properly.

[ryan-berger]

Just finished ClassLink up. Code available in #421 🎉