Add parseDocument, which builds a full HTML document by the HTML5
tree-construction insertion modes (initial, before html, before head, in head,
after head, in body):
- the result always has an html root containing a head and a body, whether or
not the source spelled those elements out.
- head-only elements (title, meta, link, base, style, script, and the rest)
are routed into the head; flow content starts and fills the body.
- explicit html, head, and body tags and their attributes are honored.
The existing fragment parser (parse) is unchanged, so tag-soup fragments still
round-trip without an implied wrapper.
Closes#69
