Emit mw:TemplateParams info for every tpl-wrapped DOM subtree.

* For every DOM subtree that is wrapped as an atomic template
  (even if content comes from multiple template transclusions),
  emit a mw:TemplateParams meta-object that specifies template
  params for every template transcluded into the wrapped DOM.

* Updated tokenizer to record wikitext source offsets for
  template parameter wikitext at transclusion sites.

Change-Id: I65417d480678a65e32d4e78bfed937a31cc53964