Add support for US territories

[pdil]

A highly requested feature is the ability for usmap to optionally include US territories (e.g. Puerto Rico) in its map data and plots.

This will require changes to create-us-map.R to generate the maps and us-map.R to retrieve the map given the territory parameters.

The work for this is also tracked in the usmap repo: pdil/usmap#20

[pdil]

The Census cartographic boundary files only contain Puerto Rico (besides the US states & DC). To include other territories usmapdata would have to use the TIGER/Line Shapefiles, the file sizes of which are way too large to include in this package. As such only the inclusion of Puerto Rico will be considered at this time until another solution is found for the other territories.