Use model_key for insert in strict mode and add initial_version_key

[ottobar]

The model key was hardcoded to :model in the strict mode branch of PaperTrail.Multi.insert/3 and was modified to use the model_key variable.

[ottobar]

Also realized that :initial_version was hardcoded, so added an option for that as well.

[izelnakri]

Hi @ottobar , looks like a nice contribution, thank you! I’ll pull, review and test this the next 2 days, meanwhile feel free to add yourself to the contributors :)