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.

src/Spout/Writer/Common/Manager/Style/StyleMerger.php

@@ -85,6 +85,9 @@ class StyleMerger
         if (!$style->hasSetWrapText() && $baseStyle->shouldWrapText()) {
             $styleToUpdate->setShouldWrapText();
         }
+        if (!$style->hasSetCellAlignment() && $baseStyle->shouldApplyCellAlignment()) {
+            $styleToUpdate->setCellAlignment($baseStyle->getCellAlignment());
+        }
         if (!$style->getBorder() && $baseStyle->shouldApplyBorder()) {
             $styleToUpdate->setBorder($baseStyle->getBorder());
         }