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/Common/Entity/Style/CellAlignment.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Box\Spout\Common\Entity\Style;
+
+/**
+ * Class Alignment
+ * This class provides constants to work with text alignment.
+ */
+abstract class CellAlignment
+{
+    const LEFT = 'left';
+    const RIGHT = 'right';
+    const CENTER = 'center';
+    const JUSTIFY = 'justify';
+
+    private static $VALID_ALIGNMENTS = [
+        self::LEFT => 1,
+        self::RIGHT => 1,
+        self::CENTER => 1,
+        self::JUSTIFY => 1,
+    ];
+
+    /**
+     * @param string $cellAlignment
+     *
+     * @return bool Whether the given cell alignment is valid
+     */
+    public static function isValid($cellAlignment)
+    {
+        return isset(self::$VALID_ALIGNMENTS[$cellAlignment]);
+    }
+}