Enable subtotals to be hidden by group

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:

* `displaySubtotals` (defaults to `true`).