Incorporate pybaobab

[gsganden]

I really like this tree visualization technique. I think it is overall more useful than the box-and-arrow diagrams we use currently. However, the box-and-arrow diagrams have their place, especially for explaining decision trees to beginners, which is the primary use case for which I developed model_inspector. It would be nice to support both, but I'd have to think about the interface. I like the idea of having a single show_model method that's opinionated about how best to show a model, but that approach doesn't fit nicely here.