Stylelint linter output stream can go to stderr (#4702)

Since [version 13.6.0](804bb24c75/CHANGELOG.md (L654)),
following [PR 4799](https://github.com/stylelint/stylelint/pull/4799)
`stylelint` errors are sent to `stderr`. Previous versions where sending
errors to `stdout`.

ale_linters/html/stylelint.vim

@@ -21,6 +21,7 @@ endfunction
 
 call ale#linter#Define('html', {
 \   'name': 'stylelint',
+\   'output_stream': 'both',
 \   'executable': function('ale_linters#html#stylelint#GetExecutable'),
 \   'command': function('ale_linters#html#stylelint#GetCommand'),
 \   'callback': 'ale#handlers#css#HandleStyleLintFormat',