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?
In this instance it's thankfully not necessary anymore to check it. We throw an error in htmltodom.js right now though where this is checked.

In principle parse5 can't handle xml - we have to let the user know that it's possible to do that with htmlparser2 though.

Maybe a flag like ```ignoreXML``` which - in combination with parse5 - supresses the error?