Merge pull request #15 from Automattic/convert-output-to-tree

The final output should resemeble something like a VDOM structure.

These changes recursively strip away the information from the parse and
turn it into an easier output tree of [ type, attrs, children ] where
type is a pair of [ major type, minor type ] and major type could
be block or tag (for an HTML tag).

This is just one path towards forming a language with which to
communicate with plugins. The idea is that this format is trivial to
abstract with functions.