add borderlines to plots

[refszkentla]

What problem does your feature request solve?

Well, not every country is an island. In our case, with have lots of neighbours and just a piece of coastline. It would be great to have borders as well, especially that in current plot.py it is extremely easy, with cartopy already ensured by reqs and just 2 lines, i guess

Describe the solution you'd like

1. Add import cartopy.feature as cfeature in the imports section
2. add axes.add_feature(cfeature.BORDERS, edgecolor=coastcol) in the helper _setup_spatial_map function, just below
   axes.coastlines(resolution="10m", color=coastcol) line; i think, border and coastlines can be just the same color.
   (I have tested this minor approach and it worked)

Describe alternatives you've considered

I haven't considered any alternatives.