function toLabel(row, column) {
  var rowLabel = (row.isAbsolute ? '$' : '') + rowIndexToLabel(row.index);
  var columnLabel = (column.isAbsolute ? '$' : '') + columnIndexToLabel(column.index);

  return columnLabel + rowLabel;
}
Convert row and column indexes into cell label.

@param {Object} row Object with `index` and `isAbsolute` properties.
@param {Object} column Object with `index` and `isAbsolute` properties.
@returns {String} Returns cell label.