Require a groupBy Column Option

[Neldrick]

Currently we have :
sortType: (rowA, rowB, id),
filter:(rows, id, filterValue),

in Column option, can we add groupBy: (rows, columnId) ?

which change the defaultGroupByFn inside useGroupBy with something like:

const currentColumn = allColumns.find(col => col.id === columnId) const customGroupByFn = typeof currentColumn.groupBy === 'function' ? currentColumn.groupBy : groupByFn let rowGroupsMap = customGroupByFn(rows, columnId)

This would allow user to custom there own grouping method on specific column rather than changing the whole groupByFn for all columns