move most top-level values of Omd_parser to its functor Make

Omd_parser.Make  is functor  that  takes an  environment (module  type
Omd_parser.Env)  and  returns  a  markdown  parser  that  shares  some
environment values.  This allowed to replace some top-level references
(of Omd_parser) by values passed in the module parameter (Env).

Files concerned by this change:
 - omd.ml
 - omd_main.ml
 - omd_parser.ml
 - omd_parser.mli