Get table of all cities releasing GBFS feeds
get_gbfs_cities()
North American Bikeshare Association, General Bikeshare Feed Specification https://raw.githubusercontent.com/MobilityData/gbfs/master/systems.csv
A data.frame
of all cities issuing GBFS feeds. The `Auto-Discovery URL`
column supplies the relevant .json feeds, while the entries in the `URL` column
take the user to the public-facing webpage of the programs.