Return figure in waterfall plot if show is false
Return figure in waterfall plot if show is false

Updates waterfall plot to be consistent with the (documentation)[https://github.com/slundberg/shap/blob/cb73243c98f94fbb77180a719c780156d0bddfbe/shap/plots/_waterfall.py#L37] and with (force_matplotlib)[https://github.com/slundberg/shap/blob/cb73243c98f94fbb77180a719c780156d0bddfbe/shap/plots/_force_matplotlib.py#L409]
slundberg/shap,slundberg/shap,slundberg/shap,slundberg/shap