Add option to hide totals row.

Allow groups to exclude the subtotals from the rendered grid.

This is to allow using group formatters to display aggregate information in the group header rather than in a footer section (if desired).
I'm sure there are other uses as well, I just haven't been able to think of any.

Also added a new groupingInfoOption:
* displayTotalsRow (defaults to true).

We'll see if Github is smart enough to update the existing pull request.