Add warmup_draws = T argument to read_cmdstan_csv #565

I'd like to change read_cmdstan_csv so that I can avoid reading in warmup draws even if the model was fit with save_warmup = T. Proposed fix is to add a warmup_draws = T argument to read_cmdstan_csv. There would be no change in the default behavior, but if warmup_draws is set to F the returned object would be equivalent to the current default behavior as if the model had been fit with save_warmup = F.

Any comments/objections to me adding this?