Add white slaver to list of potential subjects
Add white slaver to list of potential subjects

This change was made after a helpful discussion on Twitter. My thanks
to Tressie McMillian Cottom and L.D. Burnett for encouraging me to
say more in the tweets about the subjectivity of those buying and
selling people in the antebellum South, especially since the previous
code did sometimes refer to those bought as "a black person."

See also: https://storify.com/wcaleb/subjectivity-on-every3minutes

wcaleb/everythreeminutes