Breaking changes to Sources

Instead of having to create a new exporter object for separate sources, you may now use the same exporter object for everything, and just pass the source into the parse method.

use pandaac\Exporter\Parsers\XMLSource;
use pandaac\Exporter\Sources\StringContent;

$exporter->parse(new XMLSource(
    new StringContent('<online><player id="1" /><player id="2" /></online>')
));