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/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
+