Add **kwargs to Explainer.plot_shap_values()

[jeongyoonlee]

Proposed changes

This PR fixes #601. It adds **kwargs to Explainer.plot_shap_values() so that users can pass additional arguments to shap.summary_plot().

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
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Further comments

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[paullo0106]

LGTM!