Implement subsampling as a script (or eventually "augur subsample")

[DSequ]

[jameshadfield]

Development was happening at nextstrain/ncov#711 but it has stalled a bit

[sidneymbell]

Update: we discussed this with the Nextstrain team last week, and while this is still very much of interest to both the Nextstrain team and users, it doesn't seem like the right time to dive into this work. For more context, see the doc.

[victorlin]

Duplicate of #635