Dtreeviz

Added cool leaf viz features from tlapusan `viz_leaf_samples()` and `ctreeviz_leaf_samples`.

<img width="1066" alt="Screen Shot 2019-10-23 at 3 51 03 PM" src="https://user-images.githubusercontent.com/178777/67440098-04315500-f5ad-11e9-82dc-4da3b41f89bd.png">
<img width="1058" alt="Screen Shot 2019-10-23 at 3 51 13 PM" src="https://user-images.githubusercontent.com/178777/67440104-05fb1880-f5ad-11e9-9153-80c3b6d0d225.png">

Also, allowed ax=None now in the `rtree_*` and `ctree_*` functions.

**Fixed:**

* [indexing wrt class_names & relevant issues](https://github.com/parrt/dtreeviz/pull/53)
* [added type check and conversion for y_train parameter](https://github.com/parrt/dtreeviz/pull/51)
* [the right arrows in path to test data are not highlighted](https://github.com/parrt/dtreeviz/pull/62)

See [changes for 0.5](https://github.com/parrt/dtreeviz/milestone/6?closed=1). Thanks to Ashafix and H4dr1en for the PRs:

* Configurable colors
* Added colorbar
* code clean up

Lots of [bug fixes](https://github.com/parrt/dtreeviz/milestone/5?closed=1) but main improvement is drastically [simplified install procedure for OS X and Windows](https://github.com/parrt/dtreeviz/pull/36), thanks to gautamkarnik. Also, I merged h4nyu's PR for [remove hard coded fontname](https://github.com/parrt/dtreeviz/pull/24).

Regression univariate feature-target space


Regression bivariate feature-target space

<img width="561" alt="screen shot 2018-11-26 at 11 39 23 am" src="https://user-images.githubusercontent.com/178777/49104999-4edb0d80-f234-11e8-9010-73b7c0ba5fb9.png">

Regression bivariate feature-target space heatmap


Classification univariate feature-target space


Classification bivariate feature-target space


Just a tweak to fix [path issue for windows](https://github.com/parrt/dtreeviz/issues/12). thanks to whamp.