Ability to add processing instructions

[prestonholt]

I came across this issue #143 and also needed this feature. It adds the method addProcessingInstruction($target, $data) to the ArrayToXML object and can be used like this:

$arrayToXml = new ArrayToXml($array);
$arrayToXml->addProcessingInstruction('qbxml', 'version="13.0"');
$result = $arrayToXml->toXml();

Which would generate:

<?xml version="1.0"?>
<?qbxml version="13.0"?>
<root><Good_guy><name>Luke Skywalker</name><weapon>Lightsaber</weapon></Good_guy><Bad_guy><name>Sauron</name><weapon>Evil Eye</weapon></Bad_guy></root>

Multiple processing instructions can be added, and they will always be prepended above the root element.

I figured this may not be used enough to justify adding it to the convert() method arguments, but am happy to add it there if wanted.

[prestonholt]

Hey @freekmurze , just wanted to see if you had any feedback on this. Thanks!

[freekmurze]

Could you rebase against master to resolve the conflicts?

[prestonholt]

@freekmurze Conflicts should be resolved now

[freekmurze]

Thanks!