Feature request: support for weighted draws

[n-kall]

I think it could be useful to have a way to specify weights for draws for functions like mcmc_hist and mcmc_dens. This way when dealing with weighted draws (e.g. a posterior::draws_array object with weights from importance sampling), the same functions could be used as for raw mcmc draws without needing to resample.

As far as I understand, geom_density and geom_histogram already have aesthetic mapping for weights as an option, so it might be possible to use this directly.