Add ->paperSize()-method

[jeffreyvanhees]

As promised in #18 (reply in thread)

This PR adds a paperSize()-method on the PdfBuilder. Also wrote a test and added an example to the docs.

use Spatie\LaravelPdf\Facades\Pdf;

Pdf::view('pdf.receipt', ['order' => $order])
 ->paperSize(57, 500, 'mm')
 ->save('/some/directory/receipt-12345.pdf');

[freekmurze]

Thanks!