Option to return formatted dates instead of PHP objects (#226)

When reading spreadsheets, Spout should be able to return formatted dates, as shown when opened with Excel for instance.
It currently only returns DateTime/DateInterval objects, making it impossible to read + write, as the Writer does not accept objects.

src/Spout/Reader/XLSX/Sheet.php

@@ -25,12 +25,13 @@ class Sheet implements SheetInterface
      * @param string $filePath Path of the XLSX file being read
      * @param string $sheetDataXMLFilePath Path of the sheet data XML file as in [Content_Types].xml
      * @param Helper\SharedStringsHelper Helper to work with shared strings
+     * @param bool $shouldFormatDates Whether date/time values should be returned as PHP objects or be formatted as strings
      * @param int $sheetIndex Index of the sheet, based on order in the workbook (zero-based)
      * @param string $sheetName Name of the sheet
      */
-    public function __construct($filePath, $sheetDataXMLFilePath, $sharedStringsHelper, $sheetIndex, $sheetName)
+    public function __construct($filePath, $sheetDataXMLFilePath, $sharedStringsHelper, $shouldFormatDates, $sheetIndex, $sheetName)
     {
-        $this->rowIterator = new RowIterator($filePath, $sheetDataXMLFilePath, $sharedStringsHelper);
+        $this->rowIterator = new RowIterator($filePath, $sheetDataXMLFilePath, $sharedStringsHelper, $shouldFormatDates);
         $this->index = $sheetIndex;
         $this->name = $sheetName;
     }