Break shared handlers up into their own files, and fix stylelint error handling

ale_linters/haskell/hdevtools.vim

@@ -5,5 +5,5 @@ call ale#linter#Define('haskell', {
 \   'name': 'hdevtools',
 \   'executable': 'hdevtools',
 \   'command': 'hdevtools check -g -Wall -p %s %t',
-\   'callback': 'ale#handlers#HandleGhcFormat',
+\   'callback': 'ale#handlers#haskell#HandleGHCFormat',
 \})