#163 Add XML PI for xml-model

[Don Siders]

Please add support for the <?xml-model?> PI to allow association of Relax (and other) schemas with the XML document. For example:

<?xml-model href="http://www.docbook.org/xml/5.0/rng/docbook.rng" schematypens="http://relaxng.org/ns/structure/1.0"?>

<?xml-model?> is specified at: http://www.w3.org/TR/xml-model/

[Lars Huttar]

See related: https://sourceforge.net/p/xml-copy-editor/feature-requests/88/
I would rather see a more standardized PI, like <?xml-model?>, than an editor-specific one like <?xmlcopyeditor_rng_grammar?>.