Add CESTA cities/populations dataset

[lmullen]

Suggested by @hepplerj.

https://github.com/cestastanford/historical-us-city-populations

[lmullen]

Hey, @hepplerj: I added the CESTA cities data to this package in a preliminary way. Here are the details if you want to look at it.

Relevant files:

• https://github.com/ropensci/historydata/blob/master/R/us_cities_pop.R
• https://github.com/ropensci/historydata/blob/master/data-raw/cesta-cities.R

I've also taken the liberty of adding you and Steiner as contributors, which is CRAN policy:

https://github.com/ropensci/historydata/blob/master/DESCRIPTION

[hepplerj]

Looks great, thanks!