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]);
+    }
+}

src/Spout/Common/Entity/Style/Color.php

@@ -8,7 +8,7 @@ use Box\Spout\Common\Exception\InvalidColorException;
  * Class Color
  * This class provides constants and functions to work with colors
  */
-class Color
+abstract class Color
 {
     /** Standard colors - based on Office Online */
     const BLACK = '000000';

src/Spout/Common/Entity/Style/Style.php

@@ -8,7 +8,7 @@ namespace Box\Spout\Common\Entity\Style;
  */
 class Style
 {
-    /** Default font values */
+    /** Default values */
     const DEFAULT_FONT_SIZE = 11;
     const DEFAULT_FONT_COLOR = Color::BLACK;
     const DEFAULT_FONT_NAME = 'Arial';
@@ -54,6 +54,13 @@ class Style
     /** @var bool Whether specific font properties should be applied */
     private $shouldApplyFont = false;
 
+    /** @var bool Whether specific cell alignment should be applied */
+    private $shouldApplyCellAlignment = false;
+    /** @var string Cell alignment */
+    private $cellAlignment;
+    /** @var bool Whether the cell alignment property was set */
+    private $hasSetCellAlignment = false;
+
     /** @var bool Whether the text should wrap in the cell (useful for long or multi-lines text) */
     private $shouldWrapText = false;
     /** @var bool Whether the wrap text property was set */
@@ -325,6 +332,44 @@ class Style
         return $this->hasSetFontName;
     }
 
+    /**
+     * @return string
+     */
+    public function getCellAlignment()
+    {
+        return $this->cellAlignment;
+    }
+
+    /**
+     * @param string $cellAlignment The cell alignment
+     *
+     * @return Style
+     */
+    public function setCellAlignment($cellAlignment)
+    {
+        $this->cellAlignment = $cellAlignment;
+        $this->hasSetCellAlignment = true;
+        $this->shouldApplyCellAlignment = true;
+
+        return $this;
+    }
+
+    /**
+     * @return bool
+     */
+    public function hasSetCellAlignment()
+    {
+        return $this->hasSetCellAlignment;
+    }
+
+    /**
+     * @return bool Whether specific cell alignment should be applied
+     */
+    public function shouldApplyCellAlignment()
+    {
+        return $this->shouldApplyCellAlignment;
+    }
+
     /**
      * @return bool
      */