provide a way to specify a genome list independent of the genbank gather

[ctb]

(and then run gather on those genomes to get the order)

[ctb]

e.g., could do something like this:

config-.yml:

[<sample>]
ref_genomes:
- A
- B
- C

if you want to override.

also provide a CLI that lets you just download a genome by accession

and/or figure out how to divorce accession gathering from gather.csv as much as possible.