Add `voronoi_plot_2d` for bounded Voronoi diagrams

[alonfnt]

Visualizing Voronoi diagrams is one of the most common use cases, but scipy.spatial.voronoi_plot_2d doesn't handle bounded regions (unbounded edges are drawn as rays to infinity, making the output essentially unusable in practice)
Reference implementation is here.

Since boundvor already computes the clipped regions, it'd be natural to provide a plotting function that mirrors scipy's interface:

boundvor.voronoi_plot_2d(vor, ax=None)

The bounded vertices are already available in the output, so no additional bounds input should be needed.

Context: #13