[feature request] add option to add a rownumber column

[yakir12]

When debugging csv files, it's really useful to be able to talk about row numbers, e.g. "the value at column Weight, in row number 3, should not be negative".
I can achieve something like that now with:

using DataFrames
transform!(df, eachindex => :rownumber)

But this breaks apart when csv files contain empty lines (some people use empty lines to visually separate groupings more easily).