Add support for Dune (#4263)

* Add support for dune

* Add test

* Undo reformatting of Markdown file

* Refer to ale-ocaml-dune from ale.txt

autoload/ale/fix/registry.vim

@@ -78,6 +78,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['dhall'],
 \       'description': 'Standard code formatter for the Dhall language and removing dead code',
 \   },
+\   'dune': {
+\       'function': 'ale#fixers#dune#Fix',
+\       'suggested_filetypes': ['dune'],
+\       'description': 'Fix dune files with dune format',
+\   },
 \   'fecs': {
 \       'function': 'ale#fixers#fecs#Fix',
 \       'suggested_filetypes': ['javascript', 'css', 'html'],