Cell alignment

This PR adds support for cell alignment for XLSX and ODS files.
You can now align the content of the cells this way:
```
use Box\Spout\Common\Entity\Style\CellAlignment;
use Box\Spout\Writer\Common\Creator\Style\StyleBuilder;

$style = (new StyleBuilder())
    ->setCellAlignment(CellAlignment::RIGHT)
    ->build();
...
```

Possible cell alignments are: LEFT, RIGHT, CENTER and JUSTIFY.

tests/Spout/Writer/Common/Manager/Style/StyleMergerTest.php

@@ -45,6 +45,7 @@ class StyleMergerTest extends TestCase
                         ->setFontBold()
                         ->setFontColor(Color::YELLOW)
                         ->setBackgroundColor(Color::BLUE)
+                        ->setFormat('0.00')
                         ->build();
         $currentStyle = (new StyleBuilder())->setFontName('Font')->setFontUnderline()->build();
         $mergedStyle = $this->styleMerger->merge($currentStyle, $baseStyle);
@@ -60,6 +61,7 @@ class StyleMergerTest extends TestCase
         $this->assertTrue($mergedStyle->isFontUnderline());
         $this->assertEquals(Color::YELLOW, $mergedStyle->getFontColor());
         $this->assertEquals(Color::BLUE, $mergedStyle->getBackgroundColor());
+        $this->assertEquals('0.00', $mergedStyle->getFormat());
     }
 
     /**