domHelper no longer accepts argument for extensions
Initial pass at supporting namespaces, SVG

Update the simple-html-tokenizer to gain de-normalized tag and attribute
strings.

Pass the parent element into the createElement helper so the namespace
can be fetched. Fetch the namespace from dom elements as they render.
Add several svg elements to the void elements list.

This needs optimization and far more corner-case testing.
dom_helpers -> dom-helpers, use a cache for namespace discovery