Allow itemKey to be a function

[kmohrf]

Hi,

I’d be nice if itemKey would also accept a function in the form of (item: any) => number | string | symbol .
I have a list of lists and the key is deferred from the first item of that list and itemKey can’t be a string in this case, as there is no attribute of the nested list that would qualify as the key.

I might be able to put together a pull request, but maybe you’re faster :). In any case thank you for your time!

Cheers

Konrad