fn:parse-html and Validator.nu dependency

[GuntherRademacher]

These changes

• replace the dependency on TagSoup by a dependency on Validator.nu's HTML parser,
• replace TagSoup options by Validator.nu options,
• use Validator.nu for implementing html:parse,
• add function fn:parse-html, based on html:parse (but, in contrast to that, without falling back to the XML parser in case Validator.nu is unavailable).

Some slight restructuring of StandardFunc and FuncOptions was necessary to be able to produce error FODC0012.