Replace `base_annotations` with `intersection_size`

[krassowski]

base_annotations is redundant to annotations. Very often users want to customize the intersection size bar plot and they should be able to just pass upset(..., intersection_size=intersection_size(<user args>) rather than specify lengthy base_annotations=list('Intersection size'=intersection_size(<user args>)). There is no value lost as the intersection size could be moved to any place using annotations. Specifying intersection_size=FALSE (or "none") would disable it.

[krassowski]

Maybe it could use a different name from intersection_size to distinguish function creating the subplot from upset() argument as it was done for matrix. So maybe just size? Or maybe more formally size_plot (which will allow users to abbreviate to size.