Add argon linter for haskell

Argon is a monitor for cyclomatic complexity
in Haskell.

https://github.com/rubik/argon

doc/ale-haskell.txt

@@ -2,6 +2,54 @@
 ALE Haskell Integration                                   *ale-haskell-options*
 
 
+===============================================================================
+argon                                                       *ale-haskell-argon*
+
+g:ale_haskell_argon_executable                 *g:ale_haskell_argon_executable*
+                                               *b:ale_haskell_argon_executable*
+  Type: |String|
+  Default: `'argon'`
+
+  This variable can be changed to use a different executable for argon.
+
+
+g:ale_haskell_argon_options                       *g:ale_haskell_argon_options*
+                                                  *b:ale_haskell_argon_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to modify flags given to argon. This
+  should usually not bet set, unless you know what you are doing.
+  For setting the minimum cyclomatic complexity set
+  `g:ale_haskell_argon_info_level` instead.
+
+
+g:ale_haskell_argon_info_level                 *g:ale_haskell_argon_info_level*
+                                               *b:ale_haskell_argon_info_level*
+  Type: |Number|
+  Default: `4`
+
+  What minimum cyclomatic complexity to consider "info level". This also
+  sets the minimum. Everything below this will not be shown.
+  Maximum is determined by `g:ale_haskell_argon_warn_level`.
+
+
+g:ale_haskell_argon_warn_level                 *g:ale_haskell_argon_warn_level*
+                                               *b:ale_haskell_argon_warn_level*
+  Type: |Number|
+  Default: `8`
+
+  What minimum cyclomatic complexity to consider a warning.
+  Maximum is determined by `g:ale_haskell_argon_error_level`.
+
+
+g:ale_haskell_argon_error_level               *g:ale_haskell_argon_error_level*
+                                              *b:ale_haskell_argon_error_level*
+  Type: |Number|
+  Default: `12`
+
+  What minimum cyclomatic complexity to consider an error.
+
 ===============================================================================
 brittany                                                 *ale-haskell-brittany*
 

doc/ale.txt

@@ -93,6 +93,7 @@ CONTENTS                                                         *ale-contents*
     handlebars............................|ale-handlebars-options|
       ember-template-lint.................|ale-handlebars-embertemplatelint|
     haskell...............................|ale-haskell-options|
+      argon...............................|ale-haskell-argon|
       brittany............................|ale-haskell-brittany|
       ghc.................................|ale-haskell-ghc|
       hdevtools...........................|ale-haskell-hdevtools|