Treat fields argument as additional fields

[mikkmart]

... like tools::write_PACKAGES().

withr::with_tempdir({
  download.packages("cranlike", ".")
  cranlike::update_PACKAGES(fields = c("Title", "Description"))
  read.dcf("PACKAGES")
})
#>      MD5sum                             Package    Version
#> [1,] "542dc9381840cfd998fecd57fc14f560" "cranlike" "1.0.3"
#>      Imports                                               
#> [1,] "DBI, debugme, desc (>= 1.1.0), RSQLite, tools, utils"
#>      Suggests                              License      NeedsCompilation
#> [1,] "covr, mockery, testthat, withr, zip" "GPL (>= 2)" "no"            
#>      Title                               
#> [1,] "Tools for 'CRAN'-Like Repositories"
#>      Description                                                          
#> [1,] "A set of functions to manage 'CRAN'-like repositories\nefficiently."
#>      File                    Filesize
#> [1,] "cranlike_1.0.3.tar.gz" "12493"

Fixes #14.