Adds new option exporting.csv.dataFormatter

[historicbruno]

I've added this option, which allows overriding the default number format for HTML data table and exported CSV. It shouldn't change the existing default behavior, and I didn't change the strange way the local decimal point is detected (I think it could be changed pretty easily, if needed).

Also, it might be sensible to have separate formatter functions for CSV and HTML table, but this is only a single option for both.