omd_parser.ml: fix: lines starting with multiple consecutive dashes were mistaken as lists, which made the program fail
omd_parser.ml: tags h2, h3, h4, h5, h6 were missing in Omd_parser.htmltags_set
omd_lexer.ml: convert each tab to 4 spaces
omd_parser.ml: fix: star-based hr after a link definition