Support `xgboost`

[gsganden]

We currently support scikit-learn models only. Adding support for xgboost might not be too difficult and would make the library more relevant for practitioners.

[gsganden]

I think we can use Model Inspector with XGBoost by way of the xgboost library's sklearn wrapper, but I haven't confirmed that, we might not want to use the wrapper, and we might want to add some XGBoost-specific methods (e.g. learning curves).