[WIP] Add HTML validator

Currently supports a subset of validation checks and has some bugs.
The core is a RNG validator (ported from Java), so, to conform spec, we only need to update RNG schema (maintained by Mozilla).

The checks this validator lacks could be performed by schematron schema validator, but I can't find a good js implementation, so I'll have to port a hand-written java-equivalent for that checks.

There are also pluggable rng schemas (aria, microdata, rdfa) to do.