• Do not encode amps or angle brackets in HTML comments + test case.
  
• Do not emit title attribute in tags unless we have contents
  for it. This was a 'feature' of original Markdown where I've decided
  to go with what php-markdown does (tests updated).
  
• Added php-markdown tests (most/all of the regular tests pass except
  the email obfu one).
  
• Added php-markdown extra tests (all fail).
  
• Added python markdown tests (mostly pass, but marked broken).
  
• Add a way of stopping inline HTML blocks from suppressing markdown
  processing. Feature request from #catalyst-dev