simplified signature and implementation

[ethanbsmith]

simplified function signature and implementation to better support subsequent maintenance
should not be breaking change as existing usage of data.type is handled by ...

closes #343

[ethanbsmith]

just realized that the the whole section below can be further simplified to probably 1 line. Ill do some testing and add another commit

 h <- curl::new_handle()
    curl::handle_setheaders(h, Authorization = paste("Token", api.key))
    response <- curl::curl_fetch_memory(URL, h)
    response.data <- rawToChar(response$content)
    stock.data <- read.csv(text=response.data, as.is=TRUE, header=TRUE)

[joshuaulrich]

Close call... I was just about to merge! Let me know when you think this is ready.

[ethanbsmith]

should be gtg

[joshuaulrich]

Awesome, thanks!