* Don't run tidy with -xml as that fails if the input is not well-formed.

* Don't run tidy with -xml as that fails if the input is not well-formed.
  Run it with -asxhtml instead, so it will output well-formed xhtml no
  matter what the input.
* Disable tidy warnings too.
* Add a new format hook, and make tidy use it, since tidy can really only
  operate on and output complete html documents, not the body chunks
  that sanitise gets.