Update code for omd 2.0.0~alpha1

The interface of omd has already changed since 2.0.0~alpha1,
this is based on @jfrolich PR ocaml-community/omd#215
and is intended as an intermediate step to ease the port to the next
release of omd. Issue #5 will not be closed for the time being.

In particular there is a regression in the way we deal with blockquotes.
These need to be addressed anyway, see #2.

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>

Author: mseri

md2mld.opam

@@ -11,7 +11,7 @@ depends: [
   "ocaml"
   "dune" {>= "2.0"}
   "base-bytes"
-  "omd"
+  "omd" {= "2.0~alpha"}
 ]
 build: ["dune" "build" "-p" name "-j" jobs "@install" "@runtest" {with-test}]
 dev-repo: "git+https://github.com/mseri/md2mld.git"