Added render method and graph export with path variable

[Rambatino]

There's now a render method, so when running:

python -m CHAID tests/data/titanic.csv survived sex embarked --max-depth 4 --min-parent-node-size 2 --alpha-merge 0.05 --export-path /Users/Mark/Downloads/lol.gv

or

python -m CHAID tests/data/titanic.csv survived sex embarked --max-depth 4 --min-parent-node-size 2 --alpha-merge 0.05 --export

It will show:

lol.gv.pdf

This should satisfy https://github.com/Rambatino/CHAID/issues/93. I'm going to make it look a bit nicer though

Changed the chart to look like:

For easier viewing.

[Rambatino]

fixes Rambatino/CHAID/pull/93