integrate with react-contextmenu

[emacsbliss]

To follow up on #96.

I ran into issues when integrate react-contextmenu into this library.

The issue is so far seems I can only change DataViewer for the cell, but upon rendering, whatever element I returned from my custom DataViewer(which is the place I hook into react-contextmenu) is the inner children of a td element and that seems never trigger the context menu..

I felt the react-contextmenu needs to be hooked at td element level, not its children, but seems react-spreadsheet does not provide a way to change that..

thanks for the help!