Include more details about tree nodes

[tlapusan]

Nodes can contain more informations like impurity and number of samples which we can include in visualisations.

A feature request from this article : https://towardsdatascience.com/everything-you-ever-wanted-to-know-about-decision-trees-in-python-17e8edb6b37b

[parrt]

Would be good to find a way to provide that in a non-intrusive way.