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`.

doc/ale-haskell.txt

@@ -2,6 +2,16 @@
 ALE Haskell Integration                                   *ale-haskell-options*
 
 
+===============================================================================
+brittany                                                 *ale-haskell-brittany*
+
+g:ale_haskell_brittany_executable           *g:ale_haskell_brittany_executable*
+                                            *b:ale_haskell_brittany_executable*
+  Type: |String|
+  Default: `'brittany'`
+
+  This variable can be changed to use a different executable for brittany.
+
 ===============================================================================
 hdevtools                                               *ale-haskell-hdevtools*