Allow for xmlStandalone

[copierNetwork]

Added a check that $options['standalone'] = bool generates standalone declaration.

new FluidXML('books', ['standalone' => true]);
produces
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

[coveralls]

Coverage decreased (-0.4%) to 99.456% when pulling f39aa46 on copierNetwork:patch-1 into b6fb8de on servo-php:master.

[avebers]

Hello, any chance to get it merged?

[kokoshneta]

I second tridcatij’s comment. This has been lying around for over two years… can we get it merged? Being able to make standalone XML documents would be a big advantage.