Dtreeviz

Most important: Introduces XGBoost support thanks to tlapusan

Enhancements

* https://github.com/parrt/dtreeviz/pull/91
* https://github.com/parrt/dtreeviz/pull/84

Bugs:

* https://github.com/parrt/dtreeviz/pull/90

Quick enhancement: [Add scale arg to dtreeviz()](https://github.com/parrt/dtreeviz/issues/67)

Improvements

* [Display classification label on leaf node](https://github.com/parrt/dtreeviz/issues/66)

* [Make leaf split plot for regressors to show distribution of leaf sample y's]() by [Tudor Lapusan](https://github.com/tlapusan):
<img src="https://user-images.githubusercontent.com/12815158/68297044-b46b8700-009e-11ea-8e13-ecc8f569acfe.png" width="200">

* [Show just path prediction](https://github.com/parrt/dtreeviz/pull/71) by [Tudor Lapusan](https://github.com/tlapusan)

<img src="https://user-images.githubusercontent.com/12815158/69915842-2a73cb80-145c-11ea-9b68-24844e745d3d.png" width="200">

Fixes:

* [improve documentation on coloring doc enhancement](https://github.com/parrt/dtreeviz/issues/68)
* [the right arrows in path to test data are not highlighted bug](https://github.com/parrt/dtreeviz/pull/62)

* [Add histogram of leaf sizes with option to filter the leaves with less than min_samples or more than max_samples.](https://github.com/parrt/dtreeviz/pull/61)
* Added more markersize args
* don't show horizontal bars indicating prediction if no splits requested
* add new color settings: