html: create a next_node helper method

This method will handle the HTML normalization phase. That is, it
is responsible for:

  1.  closing void elements such as br and img;
  2.  inserting missing begin-tag events, such as html and body;
  3.  closing missing end-tag events, such as li and td.

This then allows the main parser in the read method to behave like
an XML parser that expects correctly balanced begin/end tag events.