Add support for header index

[Jorundur]

Like in this library: https://github.com/iuccio/csvToJson#index-header

Use case: I have a an Excel file where the 2nd row is actually the headers and the 1st row has some random data (I can't control that - I get the file from an external system). And then from row 3 onwards we have the actual data.

It would be good to be able to say "The header is row [index], and data starts below that"

[matkoniecz]

Can you process data before handing it over to parsing?