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

[tsibley]

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.

[codecov]

Codecov Report

Merging #568 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #568      +/-   ##
==========================================
+ Coverage   20.92%   21.03%   +0.10%     
==========================================
  Files          31       31              
  Lines        5133     5140       +7     
  Branches     1300     1300              
==========================================
+ Hits         1074     1081       +7     
  Misses       4007     4007              
  Partials       52       52              

Impacted Files	Coverage Δ
augur/filter.py	43.01% <100.00%> (+1.46%)	⬆️

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[kairstenfay]

Awesome, thanks for doing this, Tom!

[emmahodcroft]

This is great Tom, thank you for taking care of this!