new NCBI Nucleotide web translator

Using the Entrez Eutils JSON API. This is a translator for datasets, and therefore has some limitations, e.g. apparently no author information.
This needs to be more specific. It must only trigger on NCBI nucleotide (and possible other specific databases), so include nuccore in the URL.
`^https?://(www[-.])?ncbi[-.]nlm[-.]nih[-.]gov/nuccore/` (in Scaffold)