Add ability to add custom shapefile layers

[pdil]

User should be able to add other shape file layers superimposed on the US map (e.g. roads, rivers, temperature data, etc.).

One way this could work is as follows:

roads <- # load shape file...

plot_usmap("counties") +
  add_layer(roads) +
  add_layer(...)
# etc.

This could work by applying the same spTransform that was applied to the original US map data to the layers passed through to plot_usmap.