Add function to transform data frame to coordinates matching map

[pdil]

The function would take a data frame as input that contains two columns, the first representing longitude and the second representing latitude (column names don't matter).

The output will be a data frame containing the same data frame/coordinates except with the canonical usmap projection applied to it:

CRS("+proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m +no_defs")

Function name: usmap_proj

Related issue: #13