filter: Support ISO 8601 YYYY-MM-DD syntax for --max-date and --min-date
filter: Support ISO 8601 YYYY-MM-DD syntax for --max-date and --min-date

Using YYYY-MM-DD is much more intuitive for folks not used to Augur's
style of numeric dates, and we can easily support both.

Resolves #567.

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