Custom models

[lindeloev]

As raised in #115, #107, and elsewhere, users increasingly modify the jags_code generated by mcp but get stuck when introducing new variables. We should make it easier to do this. Maybe it will fit neatly into the custom families (#101) planned for v0.5,

• Add fields with "extra_data_params" to the custom family. Perhaps allow defining out-of-model inferred parameters using prior = list(new_param = "dnorm(...)") as a way to signal that it should be sampled.
• Include "extra_data_params" in jags_data
• Sample extra_inferred_params and show them in summary() and plot_pars() etc.
• Perhaps provide an API to let users modify fit$.internal$formula_r instead of the full jags_code so that it can be used for plot(), fitted(), pp_check(), etc.