fix: misjudged nature of HTML block in some contexts

should fix https://github.com/ocaml/omd/issues/142
avoid outputing too many consecutive empty lines

modified: omd_backend.ml
fix: HTML parsing issues

details:
 - fix: inline HTML tag followed by empty line or by nothing
   https://github.com/ocaml/omd/issues/144
 + add a few warnings, which cleans the code a little
 - fix: HTML comments followed by HTML tags
   https://github.com/ocaml/omd/issues/143

modified: omd_parser.ml