Change sortBy to have flag for sorting subRows.

[rctbusk]

Sorting performance is very slow when I have a lot of rows with sub rows. Would be nice to have a flag to disable the sorting of sub rows, or even better would be only sorting sub rows when a row is expanded.

I know this could be done by passing in my own sorting function, but having looked through the useSortBy hook, I think the use case is easy to implement and would help a lot with performance.

I understand that I could also do lazy loading for sub rows, but this gets very complicated when I am virtualizing the table.