[feature-request] Use set_inits() after it works in brms

[venpopov]

As we've discussed, for some of the models we need a way to set initial values.

I suggested to Paul a set_inits() type of function that works similarly to set_prior() and he liked the idea: paul-buerkner/brms#1645

I've made a PR with an initial prototype which works for b and Intercept parameters: paul-buerkner/brms#1646

But it will take a while to iron things out and generalize this. Once it is done, we can use this functionality for our models with a new function like "configure_inits", similarly to how we do it for default priors.