Allow selection of GenBank release?

[joelnitta]

For reproducibility, it would be nice to be able to specify a GenBank release instead of only downloading the most recent.

Example dummy code:
db_download(preselection = 20, release = 248)

This would depend of course on NCBI making old releases available somewhere... I have looked around a bit but not found anything yet.

[maelle]

For info, we're looking for a new maintainer / a new maintainer team for this package, see #23. If you read this and are a restez user, feel free to volunteer, we'd be happy to help.