adds fixer support for hfmt (#1027)

Add support for fixing Haskell with hfmt

autoload/ale/fixers/hfmt.vim

@@ -0,0 +1,16 @@
+" Author: zack <zack@kourouma.me>
+" Description: Integration of hfmt with ALE.
+
+call ale#Set('haskell_hfmt_executable', 'hfmt')
+
+function! ale#fixers#hfmt#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'haskell_hfmt_executable')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' -w'
+    \       . ' %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction
+