Custom environment table order

In Python 3.6+ or with `OrderedDict`, the config metadata can maintain its order to control the layout of the environment table. Would you consider an option to make `sorted()` optional in this line:

https://github.com/pytest-dev/pytest-html/blob/35a610065e50b94db30b068950e50d28f9c3545b/pytest_html/plugin.py#L468