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.
Removed the `ReaderEntityFactory::createReader(Type)` method and replaced it by 3 methods:
- `ReaderEntityFactory::createCSVReader()`
- `ReaderEntityFactory::createXLSXReader()`
- `ReaderEntityFactory::createODSReader()`
This has the advantage of enabling autocomplete in the IDE, as the return type is no longer the interface but the concrete type. Since readers may expose different options, this is pretty useful.
Similarly, removed the `WriterEntityFactory::createWriter(Type)` method and replaced it by 3 methods:
- `WriterEntityFactory::createCSVWriter()`
- `WriterEntityFactory::createXLSXWriter()`
- `WriterEntityFactory::createODSWriter()`
Since this is a breaking change, I also updated the Upgrade guide.
Finally, the doc is up to date too.
