omd_parser.ml: fix: lists starting with an item using a dash
omd_parser.ml: cosmetics (mostly trailing spaces and indentation)
omd_parser.ml: cosmetics (cleaning the code)
omd_backend.ml: implementing IDs generation for H1, H2 and H3
omd_backend.ml: changing id='something' to id="something"
omd_parser.ml: cf. comments (on github) on this commit