Introduce IvoryT, initially as just a Repository Reader

So we only just merge the quality guide and I'm already going against it...

> Configuration goes in as arguments. Remove mix of "configuration" styles with implicits and readers. Any reader should be carefully considered and should have non-trivial gains across the entire code base, not just the code you are writing.

I personally think it's an improvement, but now is the time to say something (see `ImportWorkflow` for one example).

/cc @raronson @etorreborre @markhibberd