Add fourmolu fixer (#4501)

* Add fourmolu fixer

Fourmolu is aversion of Ormolu that supports configuration. This fixer
was modeled after the Ormolu one, but using the "stack executable"
approach of the Brittany and Stylish Haskell fixers.

* Sort supported-tools.md

doc/ale-haskell.txt

@@ -224,5 +224,25 @@ g:ale_haskell_ormolu_options                     *g:ale_haskell_ormolu_options*
   executable.
 
 
+===============================================================================
+fourmolu                                                 *ale-haskell-fourmolu*
+
+g:ale_haskell_fourmolu_executable           *g:ale_haskell_fourmolu_executable*
+                                            *b:ale_haskell_fourmolu_executable*
+  Type: |String|
+  Default: `'fourmolu'`
+
+  This variable can be changed to use a different executable for fourmolu.
+
+
+g:ale_haskell_fourmolu_options                 *g:ale_haskell_fourmolu_options*
+                                               *b:ale_haskell_fourmolu_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be used to pass extra options to the underlying fourmolu
+  executable.
+
+
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: