Glottolog custom downloads

While the [Glottolog CLDF dataset](https://doi.org/10.5281/zenodo.3260727) serves as the main avenue to access Glottolog data in bulk, the Glottolog project has provided a couple of custom aggregations of data for download on https://glottolog.org . These files are archived in this dataset, with versions of this Zenodo deposit corresponding to versions of Glottolog.