Add an option to hide the overlay if consent has already been given

[MoritzLost]

Follow-up to the discussion in #10

Some external content may require external resources to load before it can be displayed, some will be visible almost immediately. In the first case, showing the overlay and then fading it out (the current behaviour) makes sense. But if the external content will be displayed immediately, having the overlay just show up very briefly and then fade out immediately doesn't look great, it looks more like a bug to the users.

As a solution, I suggest an optional option (either for the entire library or for individual services) to adjust this behaviour. If activated, the option should change the behaviour as followed:

• If no consent has been given for the service, the overlay should be displayed normally and fade out as it currently does.
• If consent has already been given for the service when the library is initialized, the library should skip showing the overlay and immediately reveal the external content (and call the onAccept callback).