Referrer-Policy

Thanks for your work on this plugin. This makes adding security related http headers a breeze. Another good one would be the Referrer-Policy. Below is documentation on it.

As a side note it might be handy to just give users the ability to put in their own headers, that way if there are newer headers related to security users could put in their own key/value pairs until they are added into the plugin proper. I could imagine this a simply as a custom http header section with a textbox for the key and another for the value and a button to add more.

https://developer.mozilla.org/ja/docs/Web/HTTP/Headers/Referrer-Policy