filter: Report min and max date separately

[victorlin]

Description of proposed changes

This allows for more detail in the report, and makes it more consistent by having one report line per parameter.

Motivated by work in #854.

Related issue(s)

N/A

Testing

Added cram test to verify new output.

[codecov]

Codecov Report

Merging #930 (1cdde92) into master (9dda176) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #930      +/-   ##
==========================================
+ Coverage   59.59%   59.63%   +0.03%     
==========================================
  Files          43       43              
  Lines        6056     6062       +6     
  Branches     1550     1551       +1     
==========================================
+ Hits         3609     3615       +6     
  Misses       2188     2188              
  Partials      259      259              

Impacted Files	Coverage Δ
augur/filter.py	96.35% <100.00%> (+0.04%)	⬆️

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[huddlej]

Nice! This is a great UI improvement. It's also great to see how the intended modularity of the filter functions worked out for this kind of change.