readOnly prop on the DataSheet component

[educastellano]

It would be useful to just pass a prop when you want the entire sheet to be readOnly, so it would save me to loop all the cells.

Would you accept a PR with this feature?

[jakehjung]

You can actually do this instead for a workaround:

<ReactDataSheet data={grid} valueRenderer={cell => { cell.readOnly = true; return cell.value; }} />