Call the getCellContent callback for the row markers by toggling a prop

[Druthi]

The row markers currently always show the row number as the text but I have a use case to hide some rows for which I need to show a different row number.

I want to be able to dictate the number to be shown on the row marker so if we call the getCellContent even for the rowMarker, then I can provide the text to be shown

[BrianHung]

#969 should allow you to override how row markers are drawn. Hard to solve everyone's use cases generically otherwise.