Row group hiding

A continuation of the work in #6 which introduced hideIfAllUnique and hideIfAllSame - to complete this feature I'd like to see support for:

1. Hiding the grouping based on the entire data set (unfiltered), which is what you had (although it didn't take into account any updates in the data)
2. Hiding the grouping based on the filtered data set
3. Hiding the grouping based on the current page only

For example hideIfUnique could be an array: [ 'all', 'filtered', 'page' ] for all of the above and the developer using them can selectively enable or disable the options they want.