Add background and margin on generated barcode

[osflake]

Hello Mr @casperbakker,

I have one suggestions, because when I used Your library (very very thanks for that) we found a very big problem in our 1D scanner.

When we present barcode in dark mode (night mode) web browser change background to dark and outer lines are not readable.

It is possible to add param like:

$generator->setMargin(10);

$generator = new Picqer\Barcode\BarcodeGeneratorJPG();
$generator->useGd();
$generator->setMargin(10);
$result = $generator->getBarcode($inputValue, $generator::TYPE_CODE_128, 4, 120, $color = array(0, 0, 0));    
header('Content-Type: image/png');
echo $result;

or

$setMargin = 10;
$result = $generator->getBarcode($inputValue, $generator::TYPE_CODE_128, 4, 120, $color = array(0, 0, 0),$setMargin); 

Its very important issue.

Thank You