Add get_model_priors function to access the default priors that will be used by brms for a specified model

[GidonFrischkorn]

I found it super useful to see which priors brms will specify per default for a certain model, given the data, the family, and the model. Maybe we can write a wrapper around the brms get_prior function to obtain the priors that brms would specify without any additional user input. This should give users an idea of what the default priors look like.