Change the HTML attribute prefix from html/ to html/0/

This means that <p data-foo="bar"> will now be converted to a paragraph
with attributes {"html/0/data-foo":"bar"} rather than {"html/foo":"bar"}

This paves the way for multi-element node (about group) handling in the
node API: nodes representing multiple DOM elements will have html/i/attr
to represent an attribute of the i'th DOM element.

Change-Id: Iea52bdccd721942ca708c8f9f47e934524809845
Merge "Change the HTML attribute prefix from html/ to html/0/"