XGBoost support

[KOLANICH]

see https://github.com/KOLANICH/pyxgboost for xgboost parser and how to use it.
Feel free to split that project into two: xgboost -> onnx and onnx -> python

[wschin]

Thanks a lot for your sharing. We also have XGBoost & LibSVM converters in WinMLTools. I will take a look at your package and learn from you. I personally think we should include them in ONNXMLTools.