Fix CDATA section parsing in :class:`html.parser.HTMLParser` according to
the HTML5 standard: ``] ]>`` and ``]] >`` no longer end the CDATA section.
Add private method ``_set_support_cdata()`` which can be used to specify
how to parse ``<[CDATA[`` --- as a CDATA section in foreign content
(SVG or MathML) or as a bogus comment in the HTML namespace.
