RDataset support

[mrkn]

There already been rdataset gem to make RDataset available in Ruby. It would be nice that red-datasets has the support of RDataset using that gem.

[kou]

rdataset gem depends on Daru. We don't want to depend on Daru for red-datasets gem. We're providing red-datasets-daru for Daru integration.

[heronshoes]

I think we can close this issue.

We can use Rdataset in this repo. and red-datasets-daru gem.

[kou]

Thanks!