certain options not respected when started in a supervisior

[bcardarella]

Because the child_spec delegates to start_link: https://github.com/phoenix-playground/phoenix_playground/blob/main/lib/phoenix_playground.ex#L128

certain things that start/1 does like setting the plug are never triggered https://github.com/phoenix-playground/phoenix_playground/blob/main/lib/phoenix_playground.ex#L76-L78

[wojtekmach]

Closing in favor of #26