Consider not using "http" anymore

[buhtz]

Hello,

https://github.com/pypa/readme_renderer/blob/71f7cba4ee4a7009f4c4b2fb9b878e2704481bde/readme_renderer/clean.py#L84

Today it might be a good time to remove the support of "http" in readme_renderer because of security issues. Maybe there could be a config option to opt-in it again if needed. And before doing this there should e deprecation warning period of course