Handle multivariate models with auto_prior

[benwhalley]

Hi -auto prior is really cool, but at the moment it doesn't work with multivariate models. You need to se the "resp" column in the resulting data frame by hand for it to work. E.g. I'm currently doing this:

f1 <- bf(inflexibility~trauma)
f2 <- bf(malbehaviour~inflexibility+trauma)

autopriors <- bind_rows(auto_prior(f1, df, gaussian=TRUE) %>% mutate(resp="inflexibility"), 
                        auto_prior(f2, df, gaussian=TRUE) %>% mutate(resp="malbehaviour")) 

Would be nice if it could pick up the response from the lhs of the bf