Add hook for xml and html

[Goldziher]

Please add a hook for formatting xml and html

[macisamuele]

Pull requests are always welcome.

A few caveats

• HTML format parsing and formatting can be troublesome due to general HTML not having all closing tabs
• XML formatting depends on a set of canonisation rules that could make it complicated
• we need to ensure that the same semantic content is rendered into a unique way

I won't have bandwidth to actually do it myself, but I'll be happy to review PRs.

[Goldziher]

Appreciate it. I'm afraid I too am overbooked. Maybe someone else will be game.