Support some way to specify an header that shouldn't be persisted to the session

[segevfiner]

Although not the most common, some services sometimes have HTTP headers that should be sent only for some specific requests, but not all of them, and as such shouldn't be persisted in the session. httpie currently persists all headers except for the ones starting with Content- or If-. That means such headers will also be persisted to the session, unless you take care to use --session-read-only for such requests.

It would be convenient if there was a syntax to indicate headers that shouldn't be persisted to the session and/or a configuration setting to indicate additional headers that shouldn't be persisted to the session.