Add support for ggplot2 mapping (as well as tmap) to create_map()

[elipousson]

I started a branch where I did some re-working of the create_map() function to add optional support for ggplot2 as an alternative to tmap in mapping the disparity score data: https://github.com/elipousson/sedtR/tree/map-plot-feat

The branch also includes a refactor of the create_demo_chart() function to allow more flexibility in customization. Given that create_demo_chart() uses ggplot2, I think providing a plot helper for the geospatial data that works with ggplot2 would be useful and intuitive.