Odoc markup

[dlesbre]

Add syntax color to:

• odoc tables (added in odoc 2.3),
• odoc headings with labels ({1:label heading})
• code block with attributes (used by mdx).
• also color {@ocaml attr[ blocks as ocaml instead of verbatim (previously only @{[ block were colored as ocaml).

Here is an example test comment:

(**

{1:label section}

{table
 {tr
  {td {b hello}}
  {th heading}
 }
}

{t
  | hello | there | header | text |
  | --- | :-- | :---: | --:
  | a | b | c | d
}

{@ocaml skip[
   let x = 5
]}

{@notocaml someattribute[
    this is just colored as verbatim
]}

*)