Allow applying visual properties to defined regions

[pdil]

Is your feature request related to a problem? Please describe.
Users have requested the ability to define visual properties of the defined US Census Bureau regions such as borders and fill colors.

Describe the solution you'd like
A new shape file would probably have to be constructed where each state within the region is merged into a single entity. New parameters would have to be added to plot_usmap to define which regions' visual properties should be modified and how.

Describe alternatives you've considered
None, this idea will require some refinement and planning.

Additional context
Related StackOverflow questions:

• https://stackoverflow.com/questions/72089015/how-to-fill-region-by-different-color-on-us-map-in-r
• https://stackoverflow.com/questions/72543606/how-to-make-census-region-border-thicker-on-usmap-packake-r

[pdil]

This should be easier now that the package uses sf-based map objects (see #79). Once the desired regions are defined (possibly as a function input), sf methods can be used to combine the regions as desired.