Accept ISO 8601 dates in augur filter's max-date and min-date

[kairstenfay]

Context
When providing an argument to augur filter's --max-date option, I automatically gave it an ISO 8601 date string (e.g. '2020-06-15'). Not being much of an augur user, I was surprised to learn that it only accepts float dates (e.g. 2020.46).

Description
Allow augur filter to accept ISO 8601 date strings along with date floats in a backwards compatible way.

More context can be found in this ncov PR thread.

[jameshadfield]

👍 Small typo in the link to the ncov PR where this was discussed, it's at nextstrain/ncov#401

[kairstenfay]

Ah, thank you, James. I updated the link so that it's now functional.