`fips` should be vectorized for counties

[pdil]

Actual Behavior

Currently, the following does not work:

fips("NJ", county = c("Bergen", "Hudson"))

As it outputs the following:

#> [1] "34003"
#> Warning messages:
#> 1: In if (county_ == "") { :
#>   the condition has length > 1 and only the first element will be used
#> 2: In name == county_ :
#>   longer object length is not a multiple of shorter object length
#> 3: In name == paste(county_, "county") :
#>   longer object length is not a multiple of shorter object length

It just results in the FIPS code of the first county entered in the vector, followed by a warning.

Expected Behavior

fips("NJ", county = c("Bergen", "Hudson"))
#> [1] "34003" "34017"

This would allow the ability to quickly convert a column of county names to a FIPS code in a data frame.

Things to Consider

• How to handle multiple states and multiple counties at the same time?
  For example:

fips(state = c("NJ", "NY"), county = c("Bergen", "Hudson", "Westchester")

This would not work unless there was a direct mapping between the state and county parameters. In this case it would be easier to throw an error.