Add argon linter for haskell

Argon is a monitor for cyclomatic complexity
in Haskell.

https://github.com/rubik/argon

test/handler/test_argon_handler.vader

@@ -0,0 +1,74 @@
+Before:
+  runtime ale_linters/haskell/argon.vim
+
+After:
+  call ale#linter#Reset()
+
+Execute(The argon handler should parse items correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'filename': 'src/Main.hs',
+  \     'lnum': 69,
+  \     'col': 1,
+  \     'text': 'toMicrosec: cyclomatic complexity of 13',
+  \     'type': 'E',
+  \   },
+  \   {
+  \     'filename': 'src/Main.hs',
+  \     'lnum': 202,
+  \     'col': 1,
+  \     'text': 'same: cyclomatic complexity of 8',
+  \     'type': 'W',
+  \   },
+  \   {
+  \     'filename': 'src/Main.hs',
+  \     'lnum': 503,
+  \     'col': 1,
+  \     'text': 'go: cyclomatic complexity of 7',
+  \     'type': 'I',
+  \   },
+  \ ],
+  \ ale_linters#haskell#argon#Handle(bufnr(''), [json_encode([
+  \  {
+  \    "blocks": [
+  \      {
+  \        "complexity":13,
+  \        "name":"toMicrosec",
+  \        "lineno":69,
+  \        "col":1,
+  \      },
+  \      {
+  \        "complexity":8,
+  \        "name":"same",
+  \        "lineno":202,
+  \        "col":1,
+  \      },
+  \      {
+  \        "complexity":7,
+  \        "name":"go",
+  \        "lineno":503,
+  \        "col":1,
+  \      }
+  \    ],
+  \    "path":"src/Main.hs",
+  \    "type":"result"
+  \  },
+  \ ])])
+
+Execute(The argon handler should handle empty output):
+  AssertEqual
+  \ [],
+  \ ale_linters#haskell#argon#Handle(bufnr(''), [])
+
+Execute(The argon handler should handle errors):
+  AssertEqual
+  \ [],
+  \ ale_linters#haskell#argon#Handle(bufnr(''), [json_encode([
+  \  {
+  \    "path":"src/Main.hs",
+  \    "type":"error",
+  \    "message":"3:6 Illegal datatype context (use DatatypeContexts): Show a =>",
+  \  },
+  \ ])])
+