Added posibility to create "in memory" documents

Open writer with openToMemory(), then close it with closeAndGetStream() which will return the stream pointer. After that you have to do fclose() on that pointer.

src/Spout/Writer/WriterAbstract.php

@@ -142,6 +142,33 @@ abstract class WriterAbstract implements WriterInterface
 
         return $this;
     }
+    
+    public function openToMemory()
+    {
+        $this->filePointer = $this->globalFunctionsHelper->fopen('php://temp', 'r+');
+        $this->throwIfFilePointerIsNotAvailable();
+
+        $this->openWriter();
+        $this->isWriterOpened = true;
+
+        return $this;
+    }
+
+    public function closeAndGetStream()
+    {
+        if (!$this->isWriterOpened) {
+            return;
+        }
+
+        $this->closeWriter();
+
+        $this->isWriterOpened = false;
+
+        rewind($this->filePointer);
+        fpassthru($this->filePointer);
+
+        return $this->filePointer;
+    }
 
     /**
      * Checks if the pointer to the file/stream to write to is available.