Support for model-based imputation of partially unobserved response variables

[eb8680]

brms supports joint model-based imputation by allowing some response variables to be marked partially missing using the mi() syntax:

xf <- bf(x | mi() ~ z + 1)
yf <- bf(y ~ mi(x) + z + 1)

It would be nice if brmp supported this behavior, and even turned it on by default without requiring the extra mi syntax. I believe we would have to address https://github.com/pyro-ppl/pyro/issues/1106 upstream in Pyro, after which the code generation to support this feature would be a small extension of #42.