fix: adapt to past changes in _oasis

modified:   Makefile
remove fewer `NL`s

modified: omd_representation.ml
fix: don't transform double newlines to single newlines

modified: omd_parser.ml
attempt to fix newlines policy

modified:   src/omd_backend.ml
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