use html5lib's sanitizer, supersedes 3713d5e

Python's HTMLParser is smart enough to filter malicious tags but fails
to repair invalid, user-inputted HTML. Instead of re-inventing the
wheel, Isso now uses html5lib's HTMLSanitizer with a whitelist of all
tags generated by Sundown.

Disallowed tags are discarded from the output to match the previous
unittests. This feature is only available for html5lib 0.99(9) and
later. Earlier releases just escape disallowed tags.
link example cfg, fix markup errors and show content table, closes #52

and also reword a few paragraphs as suggested via mail.