Add capability to plot counties

[blychs]

Adds capability to plot county lines to draw_map(), by adding the optional keyword counties, using NaturalEarthFeature.

The only drawback I can think of in my implementation is that, for consistency with the function structure, I added the counties parameter just before the states parameter (since countries is after states). This means that if someone is calling the function using just the position, it might go wrong (i.e., if they are doing draw_map(None, False, True, True, False, '10m') or something similar instead of draw_map(crs=None, natural_earth=True, coastlines=True) etc.). This seems to me quite unlikely, since it would render the function call almost unreadable. However, if we think it could be an issue, an easy workaround is moving the counties parameter to the end of the function, just before the kwargs.

[zmoon]

The concern about passing args positionally is valid, but, like you say, keyword usage is much more likely (all the uses within monet itself are this way).

[zmoon]

@blychs it's not related to your PR, but can you add the line ./monet/plots/taylordiagram.py:12: to the .codespell-exclude file? (to get codespell passing again)