Hybrid native/non-native versioning support#314

Hi, I would like to support both native and non-native versioning in my codebase. i.e. some models use native versioning and others don't. I tried using multiple versioning managers and specifying the version manager for each model, after making some changes in the `sqlalchemy-continuum` codebase, I was able to get the migrations to work correctly. However I am facing an issue when I update an object, the issue is that I get `sqlalchemy object` already exists for the Transaction model. I assume this is because both the versioning mechanisms are kicking in when I update an object and that causes an issue. What would you recommend as the right approach? --- My objective to use non-native versioning is for the in-app plugins (which for obvious reasons cannot be achieved in native versioning)