Consider tools for extending vetiver to custom models

[juliasilge]

I think you're right that we should consider making it easier for folks to extend vetiver to custom models. In Python, this already works because of how the handlers are set up. In R, we are using S3 methods so it's not as straightforward right now. In the longer term, we could think about how to expose an easier way to support custom models. In the shorter term, it's worth documenting what you need to do to extend vetiver for now. Some similar documentation exists for butcher and broom. Maybe we could make something like butcher's new_model_butcher() that generates a file with example code to start from.

Originally posted by @juliasilge in #214 (comment)