tidy: Add tidy-html5 as --HEAD

This build tidy from the experimental html5 fork which adds support for
validating html5 syntax.

Given that the original tidy has not been updated in ages, it should be
a relatively safe assumption that --HEAD users want the html5 version.
Also tidy-html5 has been rejected as a standalone formula, because it
has no stable release yet.

More info:
http://w3c.github.com/tidy-html5/
https://github.com/w3c/tidy-html5/