Add support to download pre-releases as well for github

ATM pre-releases are skipped

```python
            if rel.prerelease:
                log.info("Release %s is prerelease; skipping", rel.tag_name)
                continue
```

so we should add config to optionally enable them, and then enable for e.g. https://github.com/sylabs/singularity/ (on smaug:/mnt/datasets/datalad/ci/singularity)