adding option to pass existing axes to map_geodataframe_column function + corresponding test

[mjgleason]

This is a small PR that enables using the map_geodataframe_column() function to accept an existing axes instead of always plotting on a new axes. This is helpful in cases where you want to create a multipanel figure with multiple maps.

[ppinchuk]

Looks simple enough!
One thing you might consider doing, especially since it seems like the new ax option is for allowing users to create their own plots, is to add a **kwargs option to map_geodataframe_column that passes the kwargs right into gplt.polyplot. I like doing this for wrappers on plotting functions, so that you don't have to go back and change your code when users want to control customization (i.e. changing the polygon facecolor or something). Just an idea though.

[mjgleason]

Thanks @ppinchuk. That's a good suggestion, but I'm not going to take it on right now just for expediency. Will keep it on the radar for the future though!