ColorMap text color

[ThomasBur]

Addresses #130 by adding a text_color optional parameter to the ColorMap class which changes the fill: attribute of the caption and tick text.

[ThomasBur]

Hi @Conengmo thank you for the feedback. I worked out that all the places I missed for passing and docstrings where were caption was also missed. Fixed it for both of those in al spots. I got render to work, and have also tested it with folium and geopandas. Happy to make any other changes as required :)

[Conengmo]

Thanks Thomas for working on this one, much appreciated