set default value for html_input, quote the unordered_list_markers va…

[tacman]

configuration fix, so that

bin/console config:dump twig_extra > config/packages/twig_extra.yaml

now generated valid yaml and a valid configuration.

But I'm not sure that quoting the default values is correct.

                                ->defaultValue(["'-'", "'*'", "'+'"])

The YAML dumper should probably check for yaml keywords like these and quote them.