Add wrapper to get versions for a record DOI (or concept DOI)

[eblondel]

The aim is to add a simple wrapper function to get versions of a public record identified by its DOI or its concept DOI, calling the function ZenodoRecord$getVersions.

Example of execution:

get_versions("10.5281/zenodo.2547036")

Output:

        date  version                    doi
1 2019-01-22 0.1-beta 10.5281/zenodo.2547037
2 2019-06-03      0.1 10.5281/zenodo.3238351
3 2019-08-02      0.2 10.5281/zenodo.3358590
4 2019-08-27      0.3 10.5281/zenodo.3378733