Add the ability to mappify without consuming first row

[metasoarous]

If you have CSV data without a header in it, you might want to specify a header manually, and not consume the first row of the actual data (since it's not a header). Right now this could be done with

(->> ...
     (cons ["the" "header" "row"])
     (mappify))

So maybe it's not something worth worrying about. Could alias cons to add-header though, so folks don't have to think about it.