enhancement: add optional row/col paramater to to_latex formatter
I need to have to_latex formatter function know the row/col of the item being formatted, so it can modify the formatting of certain items, e.g., set some items in color of bold based on the values in the table.

To do this in a backward compatible way, I propose adding a keyword to to_latex - which would then cause the formatter function to be called as
formatter (element value, row#, col#)
