plot base customization

[jGaboardi]

Initially raised in martinfleis/sdss22-workshop#13 (this comment).

We should consider adding functionality for providing plotting keywords to the spatial weights base objects themselves (edge color, etc.), among other types of plots.

can we override how splot styles those polygons?

This is something that was partially addressed last year for plotting LISA from esda (though the flexibility for a user to set edge colors themselves is not present). However, this does not extend to plot_spatial_weights.

[jGaboardi]

A PR in libpysal seems to have slipped past us (xref pysal/libpysal#231)

[martinfleis]

Hmm, not sure if it works that easily, the two methods are doing it a bit differently. Need to look into that in detail and figure out a way to retain the flexibility.