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.
This commit introduces Row and Cell entities, that will replace the arrays passed in previously.
It also adds support for Cell styling (instead of Row styling only).
