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/Creator/Style/StyleBuilder.php

@@ -3,7 +3,9 @@
 namespace Box\Spout\Writer\Common\Creator\Style;
 
 use Box\Spout\Common\Entity\Style\Border;
+use Box\Spout\Common\Entity\Style\CellAlignment;
 use Box\Spout\Common\Entity\Style\Style;
+use Box\Spout\Common\Exception\InvalidArgumentException;
 
 /**
  * Class StyleBuilder
@@ -122,6 +124,25 @@ class StyleBuilder
         return $this;
     }
 
+    /**
+     * Sets the cell alignment.
+     *
+     * @param string $cellAlignment The cell alignment
+     *
+     * @throws InvalidArgumentException If the given cell alignment is not valid
+     * @return StyleBuilder
+     */
+    public function setCellAlignment($cellAlignment)
+    {
+        if (!CellAlignment::isValid($cellAlignment)) {
+            throw new InvalidArgumentException('Invalid cell alignment value');
+        }
+
+        $this->style->setCellAlignment($cellAlignment);
+
+        return $this;
+    }
+
     /**
      * Set a border
      *