Add support for preprocessors in the ocaml fragments of mdx files

[Mathieu Barbin]

I'm trying to use ppx annotations in the ocaml sections of an mdx file, and cannot figure out where to configure it. It seems there is no preprocess field in the (mdx) stanza (looking at 0.4).

```ocaml
let f () = [%sexp "Hello, Sexp!"]
```
```mdx-error
Line 1, characters 14-18:
Error: Uninterpreted extension 'sexp'.
```

On the other hand, using a @@deriving annotation in a type declaration doesn't trigger an error, but seems to be simply ignored.

```ocaml
# type t = int [@@deriving sexp_of]
type t = int
```

Do you confirm this is not supported at the moment? Thank you