Support for creating one file per model

[Thomasdezeeuw]

I'm wondering if this project would be open to adding support to generate one file per model, instead of the current design where all models are written to a single file.

This use case comes from the fact that our dependencies/build system aren't very good at dead code elimination, which means that using only a single generated type add a couple of kBs to our bundle size, even though we don't use most of the types yet.