Add brittany for Haskell formatting

`brittany` is one of the options for Haskell source formatting. This
adds the necessary fixer files and documentation to support `brittany`
in `ALE`.

autoload/ale/fix/registry.vim

@@ -134,6 +134,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['haskell'],
 \       'description': 'Fix Haskell files with hfmt.',
 \   },
+\   'brittany': {
+\       'function': 'ale#fixers#brittany#Fix',
+\       'suggested_filetypes': ['haskell'],
+\       'description': 'Fix Haskell files with brittany.',
+\   },
 \   'refmt': {
 \       'function': 'ale#fixers#refmt#Fix',
 \       'suggested_filetypes': ['reason'],

autoload/ale/fixers/brittany.vim

@@ -0,0 +1,15 @@
+" Author: eborden <evan@evan-borden.com>
+" Description: Integration of brittany with ALE.
+
+call ale#Set('haskell_brittany_executable', 'brittany')
+
+function! ale#fixers#brittany#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'haskell_brittany_executable')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction
+