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

ale_linters/html/htmlhint.vim

@@ -28,5 +28,5 @@ call ale#linter#Define('html', {
 \   'name': 'htmlhint',
 \   'executable_callback': 'ale_linters#html#htmlhint#GetExecutable',
 \   'command_callback': 'ale_linters#html#htmlhint#GetCommand',
-\   'callback': 'ale#handlers#HandleUnixFormatAsError',
+\   'callback': 'ale#handlers#unix#HandleAsError',
 \})

ale_linters/html/proselint.vim

@@ -5,5 +5,5 @@ call ale#linter#Define('html', {
 \   'name': 'proselint',
 \   'executable': 'proselint',
 \   'command': 'proselint %t',
-\   'callback': 'ale#handlers#HandleUnixFormatAsWarning',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
 \})