acs_download: Download ACS Summary File Data
In austensen/acssf: Download ACS Summary File Data and Build Database

This function downloads American Community Survey (ACS) Summary File (SF) data from the Census Bureau's FTP site.

1	acs_download(year, span, geo, acs_dir = ".", overwrite = FALSE)

`year`	[`integer(1)`]: The year of the ACS sample. 2005 through 2016 are available.
`span`	[`integer(1)`]: The span of years for ACS estimates. ACS 1-year, and 5-year surveys are supported.
`geo`	[`character(1)`]: The 2-letter abbreviation for the state for which data will be downloaded. For geogrpahies that do not nest within states, use `"US"`.
`acs_dir`	[`character(1)`]: The root directory in which all the ACS SF data will be saved. Defaluts to current working directoy.
`overwrite`	[`logical(1)`]: Whether existing versions of these files be overwriten. Defaults to `FALSE`.