add print_figure_kwargs to whatsnew

docs/source/whatsnew/pr/print-figure-kwargs.rst

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+* added ``InlineBackend.print_figure_kwargs`` to allow passing keyword arguments
+  to matplotlib's ``Canvas.print_figure``. This can be used to change the value of
+  ``bbox_inches``, which is 'tight' by default, or set the quality of JPEG figures.