Allow use of existing mpdf instance

Added a parameter to $argOptions to pass an existing mpdf instance to method generatePdf.

Example:
$mpdf = $mpdfService->getMpdf();
$mpdf->showImageErrors = true;
$mpdfService->generatePdfResponse($html, array('mpdf'=>$mpdf));
Allow use of preexisting mpdf instance

Added a parameter to $argOptions to pass an existing mpdf instance to method generatePdf.

Example:
$mpdf = $mpdfService->getMpdf();
$mpdf->showImageErrors = true;
$mpdfService->generatePdfResponse($html, array('mpdf'=>$mpdf));
Merge pull request #6 from sirgix/master

Allow use of preexisting mpdf instance