Allow specifying an environment when calling launch_shinystan

From @paul-buerkner in #109:

My idea was that launch_shinystan could have an env (or similar named) argument to specify the environment to look for the objects. With regard to brms, I wouldn't require users to specify this argument. Rather, it would be set automatically be the function calling shinystan, e.g:

launch_shiny.brmsfit <- function(x, rstudio = getOption("shinystan.rstudio"), ...) {
  y <- model.response(model.frame(x))
  y_rep <- predict(x, summary = FALSE)
  shinystan::launch_shinystan(x$fit, rstudio = rstudio, env = environment(), ...)
}

I am not sure though, if this is the best way to find y and y_rep computed within the function calling shinystan.

Start with the launch_shinystan entry point and the launch_shiny.brmsfit example quoted in the issue. Determine how an environment should be passed to locate objects computed by the calling function, and define the behavior for callers that do not supply env. Done means launch_shinystan supports the intended environment forwarding without requiring brms users to set it manually.