(vim) Split the ocaml filetype into several filetypes

[undu]

Now ocamlinterface ocamllex and menhir types must be taken into account as well.

The filetypes are introduced by https://github.com/ocaml/vim-ocaml/pull/61

This is a draft PR to attract comments and suggestions on how to adapt the code to the change, Merlin loads under the new filetypes and syntastic works as well, ctrlp and neomake code is untested.