XML compat / parse5

I took some effort to get parse5 working with jsdom.

I put parse5 as default parser now, however since parse5 doesn't support XML, we need to make note of the parser config to re-enable htmlparser2.

There are currently test failures because I used parse5's document-only parse mode. There is a fragment-only mode, however that'd require us to still do the wrapping stuff. I guess we'll have to go through all the failing tests and check which ones of them actually produce the correct result. The alternative would be to introduce a new config option which enables/disables fragment mode.

Any general comments before I continue? @domenic?
Scripts are added [like this](https://github.com/Sebmaster/jsdom/commit/475e35c950df23838bb834fc25edcdef6852ffd8#diff-b1f8b3077f0d537329f8d71c28ee36efR4) in SVG. I'm not advocating supporting external scripts in document like this... yet, however since this test doesn't really cost us anything I think we should support it like this for now until we have sane SVG support in place.