Split the ocaml filetype into several filetypes

Now ocamlinterface ocamllex and menhir types must be taken into account
as well. Menhir filetype is not included as there seem to be no plans to
include it and even then it would take a big effort.

The filetypes are introduced by
ocaml/vim-ocaml#61

vim/merlin/dune

@@ -13,9 +13,12 @@
         (ftdetect/merlin.vim as vim/ftdetect/merlin.vim)
         (ftplugin/merlin.vim as vim/ftplugin/merlin.vim)
         (ftplugin/ocaml.vim as vim/ftplugin/ocaml.vim)
+        (ftplugin/ocamlinterface.vim as vim/ftplugin/ocamlinterface.vim)
+        (ftplugin/ocamllex.vim as vim/ftplugin/ocamllex.vim)
         (ftplugin/omlet.vim as vim/ftplugin/omlet.vim)
         (ftplugin/reason.vim as vim/ftplugin/reason.vim)
         (plugin/merlin.vim as vim/plugin/merlin.vim)
         (syntax_checkers/ocaml/merlin.vim as vim/syntax_checkers/ocaml/merlin.vim)
+        (syntax_checkers/ocamlinterface/merlin.vim as vim/syntax_checkers/ocamlinterface/merlin.vim)
         (syntax_checkers/omlet/merlin.vim as vim/syntax_checkers/omlet/merlin.vim)
         (syntax/merlin.vim as vim/syntax/merlin.vim)))

vim/merlin/ftplugin/menhir.vim

@@ -0,0 +1,2 @@
+" Activate merlin on current buffer
+call merlin#Register()

vim/merlin/ftplugin/ocamlinterface.vim

@@ -0,0 +1,2 @@
+" Activate merlin on current buffer
+call merlin#Register()

vim/merlin/ftplugin/ocamllex.vim

@@ -0,0 +1,2 @@
+" Activate merlin on current buffer
+call merlin#Register()

vim/merlin/syntax_checkers/ocamlinterface/merlin.vim

@@ -0,0 +1,23 @@
+" Enable Syntastic support
+" Make sure syntax_checkers directory is on runtime path, then set
+" :let g:syntastic_ocamlinterface_checkers=['merlin']
+
+function! SyntaxCheckers_ocamlinterface_merlin_IsAvailable()
+  if !exists("*merlin#SelectBinary")
+    return 0
+  endif
+  try
+    let l:path = merlin#SelectBinary()
+    return executable(l:path)
+  catch
+    return 0
+  endtry
+endfunction
+
+function! SyntaxCheckers_ocamlinterface_merlin_GetLocList()
+  return merlin#ErrorLocList()
+endfunction
+
+call g:SyntasticRegistry.CreateAndRegisterChecker({
+    \ 'filetype': 'ocamlinterface',
+    \ 'name': 'merlin'})