Add test for proper handling of self-closing tags

Self-closing of tags is ignored in HTML5 but not in XML.
Add xml parsing API support

This introduces a new property on the options parameters called
parsingMode. This can be set to auto (currently interpreted as html),
html or xml.

Depending on parsingMode we should validate the document and use
XmlDocument instead of HTMLDocument.

Currently the only difference is that jsdom will automatically determine
the used parser by looking at the parsingMode instead of implying xml
parsing with a <?xml directive and htmlparser2 as the configured parser.