Migrate to sf-based map data

[pdil]

• usmapdata was updated to produce sf-based objects in version 0.2.0.
• These changes update usmap to make use of these changes by using the temporary as_sf parameter in the usmapdata calls.
  • Once the migration is complete, another update will be released that removes these parameters as the new default behavior will be the sf-based objects.
• usmap_transform() has also been updated to transform inputs to match the new sf-based map data
• Updated README examples to use new sf-based data

Notes

• Plotting the raw data can be done using ggplot2::geom_sf() rather than ggplot2::geom_point().
  • There is no need to supply x and y coordinates to the aesthetic mappings.

resolves #31