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/ODS/WriterWithStyleTest.php

@@ -4,6 +4,7 @@ namespace Box\Spout\Writer\ODS;
 
 use Box\Spout\Common\Entity\Row;
 use Box\Spout\Common\Entity\Style\Border;
+use Box\Spout\Common\Entity\Style\CellAlignment;
 use Box\Spout\Common\Entity\Style\Color;
 use Box\Spout\Common\Entity\Style\Style;
 use Box\Spout\Reader\Wrapper\XMLReader;
@@ -204,6 +205,25 @@ class WriterWithStyleTest extends TestCase
         $this->assertFirstChildHasAttributeEquals('wrap', $customStyleElement, 'table-cell-properties', 'fo:wrap-option');
     }
 
+    /**
+     * @return void
+     */
+    public function testAddRowShouldApplyCellAlignment()
+    {
+        $fileName = 'test_add_row_should_apply_cell_alignment.xlsx';
+
+        $rightAlignedStyle = (new StyleBuilder())->setCellAlignment(CellAlignment::RIGHT)->build();
+        $dataRows = $this->createStyledRowsFromValues([['ods--11']], $rightAlignedStyle);
+
+        $this->writeToODSFile($dataRows, $fileName);
+
+        $styleElements = $this->getCellStyleElementsFromContentXmlFile($fileName);
+        $this->assertCount(2, $styleElements, 'There should be 2 styles (default and custom)');
+
+        $customStyleElement = $styleElements[1];
+        $this->assertFirstChildHasAttributeEquals('end', $customStyleElement, 'paragraph-properties', 'fo:text-align');
+    }
+
     /**
      * @return void
      */