Allow adapters to be an instance which is configured

[tpendragon]

Right now for ffmpeg I have to pass options as part of the create call, which means that if I switch adapters I have to change all instances of create which kind of defeats the purpose of adapters. I wonder if there's a way I can configure an adapter and register it or something so I can just change what that short-name refers to if I change my adapter?

[cjcolvar]

This is a good idea but a different code architecture. I'll have to think about this one some more.