text: Title: Support for XGBRanker
  Body: [XGBRanker](https://xgboost.readthedocs.io/en/latest/python/python_api.html#xgboost.XGBRanker) should be a very simple add; similar to #173 
  
  It should hopefully just involve (1) either using or slightly tweaking the converter for XGBRegressor (2) writing tests and making sure they pass