Row objects and Cell styling

This commit introduces Row and Cell entities, that will replace the arrays passed in previously.
It also adds support for Cell styling (instead of Row styling only).

tests/Spout/Writer/Common/Manager/CellManagerTest.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace Spout\Writer\Common\Manager;
+
+use Box\Spout\Writer\Common\Creator\Style\StyleBuilder;
+use Box\Spout\Writer\Common\Entity\Cell;
+use Box\Spout\Writer\Common\Manager\CellManager;
+use Box\Spout\Writer\Common\Manager\Style\StyleMerger;
+use PHPUnit\Framework\TestCase;
+
+class CellManagerTest extends TestCase
+{
+    /**
+     * @return void
+     */
+    public function testApplyStyle()
+    {
+        $cellManager = new CellManager(new StyleMerger());
+        $cell = new Cell('test');
+
+        $this->assertFalse($cell->getStyle()->isFontBold());
+
+        $style = (new StyleBuilder())->setFontBold()->build();
+        $cellManager->applyStyle($cell, $style);
+
+        $this->assertTrue($cell->getStyle()->isFontBold());
+    }
+}