Add Haskell IDE Engine (hie) support (#1735)

* Adding support for haskell-ide-engine
* Work with the current directory if no stack.yaml file is found
* Added Cabal file detection, updated documentation and added tests
* Updated help

test/command_callback/test_haskell_hie_callbacks.vader

@@ -0,0 +1,27 @@
+Before:
+  call ale#assert#SetUpLinterTest('haskell', 'hie')
+
+  Save &filetype
+  let &filetype = 'haskell'
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The language string should be correct):
+  AssertLSPLanguage 'haskell'
+
+Execute(The default executable should be correct):
+  AssertLinter 'hie',
+  \ ale#Escape('hie') . ' --lsp'
+
+Execute(The project root should be detected correctly):
+  AssertLSPProject g:dir
+
+  call ale#test#SetFilename('hie_paths/file.hs')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/hie_paths')
+
+Execute(The executable should be configurable):
+  let g:ale_haskell_hie_executable = 'foobar'
+
+  AssertLinter 'foobar', ale#Escape('foobar') . ' --lsp'