omd_{lexer,parser}.ml: + elimination of Return and Returns in type 'a Omd_lexer.t.  All newline styles are now converted on the fly to Unix style by the lexer.
+ cosmetics in parser.ml to prepare the ground for the implementation of the extension mechanism.
omd.mli: implementation of a parser extension mechanism.
omd.ml: implementation of a parser extension mechanism.
omd_parser.ml: implementation of a parser extension mechanism.