[python] use kwargs in tree plotting functions (#1630)
[python] use kwargs in tree plotting functions (#1630)

* use kwargs in tree plotting functions

* relaxed version

microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM