Libxml upgrade

Creating a generic interface between Tritium and Gokogiri (https://moovweb.atlassian.net/browse/PFST-49)
This is resetting state on ctx.HtmlTransformer.  It would be better to wrap this in a defer statement, to protect against any panics.  So something like "defer  func(){ ctx.HtmlTransformer = prevxform }()"