[ocaml] support eliom files

Eliom/eliomi is ocsigen's extensions for its files using the eliom ppx.

More info at http://ocsigen.org/eliom/6.6/manual/ppx-syntax

modules/lang/ocaml/config.el

@@ -113,4 +113,9 @@
     (setq +format-with 'ocp-indent)
     (when (and (executable-find "ocamlformat")
                (locate-dominating-file default-directory ".ocamlformat"))
+      (let ((ext (file-name-extension buffer-file-name t)))
+        (cond ((equal ext ".eliom")
+               (setq-local ocamlformat-file-kind 'implementation))
+              ((equal ext ".eliomi")
+               (setq-local ocamlformat-file-kind 'interface))))
       (setq +format-with 'ocamlformat))))