Save tarball as artifact on CircleCI

[zkamvar]

As part of #204, to help facilitate reviews for those who don't have the capability of installing docker or R, saving the tarballs as temporary artifacts on CircleCI would be useful for reviewers to download and inspect the output.

[hlapp]

We could accomplish this with CircleCI v2 (which allows saving "artifacts").

This would really only make sense for pull requests, because otherwise (i.e., downloading the website) we have downloading https://github.com/NESCent/popgenInfo/archive/gh-pages.zip already built into Github.