Submit to CRAN

[nmpeterson]

Potential issues to iron out first:

• Package is large (~75MB) because it is basically a data repository. They should allow it, but if not the package could be updated to replaces the Census geoms with functions that fetch and cache those datasets using tigris (i.e. just adapt the code in data-raw/load_census_api.R)
• Some examples (block_sf, blockgroup_sf, xwalk_blockgroup2cca) are very slow because they either (a) use ggplot to create maps of complex geometries, or (b) summarize large tables. Any slow xwalk_* examples should be migrated into a vignette (as part of Create vignettes #4). Hopefully there is a faster method for mapping blocks & block groups (tmap?), but if nothing is fast enough, \dontrun{} can be a last resort.