Add API to manually update database before writing files

[mm21]

Hello,

My Python package doesn't work well with static analysis since I'm setting __all__ programmatically at runtime. I also have docstrings which are dynamically modified upon import, avoiding a lot of maintenance.

I found this project to be a great fit otherwise, and a nice improvement over AutoAPI. I was able to handle my use case by adding a simple callback which is invoked after the database is built, but before files are generated. In this callback I can fix all, update docstrings, etc.

Thanks for considering! I'd be happy to cleanup / add tests if this change is acceptable.