Move shared strings caching strategy into its own component

This will help implementing different caching strategies:
- file based
- in-memory

src/Spout/Common/Helper/GlobalFunctionsHelper.php

@@ -106,6 +106,19 @@ class GlobalFunctionsHelper
         return fputcsv($handle, $fields, $delimiter, $enclosure);
     }
 
+    /**
+     * Wrapper around global function fwrite()
+     * @see fwrite()
+     *
+     * @param resource $handle
+     * @param string $string
+     * @return int
+     */
+    public function fwrite($handle, $string)
+    {
+        return fwrite($handle, $string);
+    }
+
     /**
      * Wrapper around global function fclose()
      * @see fclose()