Add support for Pipelines and Random Forests to visualize_decision_tree

[ParthivNaresh]

Currently visualize_decision_tree supports only Decision Trees. We can expand the functionality to include support for Pipelines (so that the associated estimator can be taken and visualized) and for Random Forests.

Regarding Random Forests, we can choose to add the ability to view the entire forest, or to specify a tree in the forest, or both.

[dsherry]

Lightgbm also has a plot method https://lightgbm.readthedocs.io/en/latest/pythonapi/lightgbm.plot_tree.html

Perhaps xgboost does too

[freddyaboulton]

@gsheni Let us know about this: https://gitlab.tue.nl/20040367/pybaobab