#1486713: Add a tolerant mode to HTMLParser.

#1486713: Add a tolerant mode to HTMLParser.

The motivation for adding this option is that the the functionality it
provides used to be provided by sgmllib in Python2, and was used by,
for example, BeautifulSoup.  Without this option, the Python3 version
of BeautifulSoup and the many programs that use it are crippled.

The original patch was by 'kxroberto'.  I modified it heavily but kept his
heuristics and test.  I also added additional heuristics to fix #975556,
#1046092, and part of #6191.  This patch should be completely backward
compatible:  the behavior with the default strict=True is unchanged.