Read color odering directly in `augur export v2`

[jameshadfield]

(Some) pipelines maintain a color ordering TSV (measles example) which is then combined with a colour scale (always the canonical nextstrain one as far as I'm aware) via a rule / script. This colors.txt file (deme + hex) is then used by augur export.

It would be much simpler if augur export v2 just read the color ordering directly. This is probably best as a colors.txt file without hexes, but you could also extend the auspice-config JSON to provide a scale without hexes.

[joverlee521]

Consider if we'd like to make colors a separate command as proposed in #1185. Although based on my previous look into the color scheme files, I think the main difference between pathogens is just the color ordering.