Support custom disks saving

[erikn69]

Closes #908 It could be used for cloud disks Example

// keeps old functionality
$pdf->save('relative_path/file.pdf');
$pdf->save('/absolute_path/file.pdf');
// adds new functionality
$pdf->save('file.pdf', 's3');
$pdf->save('file.pdf', 'local');
// concatenate
$pdf->save('file.pdf', 'local')->save('file.pdf', 's3');