Make it easier to manually interact with dataverse

[andybega]

1. Get the dataverse URLs: get_doi() does this in principle, but it's not the most intuitive name. Maybe something like dataverse_url(repo)?
2. Listing what's in the repos, basically some wrapper around dvn_state() with some friendlier output.
3. Convenience function for downloading/unzipping data files? I think data verse::get_file() returns a binary blob, rather than directly reading the CSV data.