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/XLSX/Manager/Style/StyleManager.php

@@ -249,9 +249,16 @@ EOD;
 
             $content .= sprintf(' applyBorder="%d"', $style->shouldApplyBorder() ? 1 : 0);
 
-            if ($style->shouldWrapText()) {
+            if ($style->shouldApplyCellAlignment() || $style->shouldWrapText()) {
                 $content .= ' applyAlignment="1">';
-                $content .= '<alignment wrapText="1"/>';
+                $content .= '<alignment';
+                if ($style->shouldApplyCellAlignment()) {
+                    $content .= sprintf(' horizontal="%s"', $style->getCellAlignment());
+                }
+                if ($style->shouldWrapText()) {
+                    $content .= ' wrapText="1"';
+                }
+                $content .= '/>';
                 $content .= '</xf>';
             } else {
                 $content .= '/>';