Adds range rendering

[thielium]

In certain situations, consumers benefit from being able to control how virtualized items are placed, such as when managing aria attributes (where the DOM hierarchy determines accessibility; see
https://www.w3.org/TR/wai-aria-1.1/#grid).

In this pull request, one could fulfill the accessibility requirement with a cellRangeRenderer function, such as the example below:

  const accessibleCellRangeRenderer = ({
    rowStartIndex,
    rowStopIndex,
    columnStartIndex,
    columnStopIndex,
    childFactory,
  }) => {
    const rows = [];
    for (let rowIndex = rowStartIndex; rowIndex <= rowStopIndex; rowIndex++) {
      const rowChildren = [];
      for (
        let columnIndex = columnStartIndex;
        columnIndex <= columnStopIndex;
        columnIndex++
      ) {
        const cell = childFactory({ rowIndex, columnIndex });
        rowChildren.push(cell);
      }
      rows.push(<div role="row">{rowChildren}</div>);
    }

    return rows;
  };

Should address #217

[bvaughn]

This code change seems solid.

I need to think on it more before I am willing to commit to merging it though. Getting rid of the range renderer prop felt like a nice change from react-virtualized to react-window. Re-adding it would add extra function calls to relatively hot code.

[thielium]

Closing -- no time to work on this, and the iron's no longer hot.