Allow custom sheet name in the XLSX writer

Added setter
Added test
Updated README

src/Spout/Writer/Internal/XLSX/Worksheet.php

@@ -97,7 +97,7 @@ EOD;
     public function getId()
     {
         // sheet number is zero-based, while ID is 1-based
-        return $this->externalSheet->getSheetNumber() + 1;
+        return $this->externalSheet->getNumber() + 1;
     }
 
     /**