export v2: Allow periods (.) in accession numbers
export v2: Allow periods (.) in accession numbers

Periods are used to separate the unversioned accession number from the
accession's revision.

I'm not sure we should be restricting this at all instead of merely
treating accessions as opaque database-specific ids, but that's a larger
change which maybe warrants more discussion first.

Resolves #831.

nextstrain/augur,nextstrain/augur,nextstrain/augur,blab/nextstrain-augur