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.