Add support for passing check_additivity argument

[Matgrb]

Closes: #102

[timvink]

Nice work Mateusz!

The PR will add support for the check_additivity argument, but users will keep running into the issue, have to lookup the docs, and change that settings.

Tools add more value if they can offload some cognitive overhead. Can we explicitly add approximate and check_additivity parameters (next to the **shap_kwargs) ? And then set a reasonable default? Which is probably check_additivity=True and explain in docs that to speed up you can set it to false.

[Matgrb]

The main difficulty here is that all shap_kwargs need to be passed to the __init__ method of Explainer, while these two are passed only to shap_values of the TreeExplainer. Explainer is transformed into TreeExplainer at initialization.
I agree that putting everything into **shap_kwargs would be easier, but also it would be harder to ensure correctness, because we would need to make sure that any parameter passed is put either to init or shap_values for all explainers.
What do you think? I will merge this one, and let's continue discussion how we can improve it.