Add ability to set cachedHighestRow/HighestColumn

When executing $objWriter->generateSheetData(), html tables are sometimes generated with way too many columns and rows.  It would be nice to override this somehow.  Being able to update the highest column and row is one solution.  Another possible solution would be passing parameters to generateSheetData($worksheetDimensions).

	$objWorksheet = $objPHPExcel->getSheet($i);
	$objWorksheet->setCachedHighestColumn($maxCol);
	$objWorksheet->setCachedHighestRow($maxRow);
	$objWriter->setSheetIndex($i);
	$html = $objWriter->generateSheetData();