v2.1.0

-Add admin1_choropleth function which allows you to create an Administrative Level 1 choropleth of any
country in the world. (Administrative Level 1 is the generic term for states, provinces, etc.). Create
a vignette demonstrating usage of this function.
-Add function county_zoom_choropleth, which allows you to zoom in on counties, not states. If an individual county is specified, render it with no fill color.
-Updated and corrected the values of df_president. Thank you to Richard Careaga.
-Update vignettes about US maps. Specifically, provide information about custom scales to include warning about interactions with insets of AK and HI.