subsample: Add simple --output-log support

[victorlin]

Description of proposed changes

This simple change adds an option to write the output log file of the final augur filter call as-is. It exposes implementation details, but could be useful for debugging.

Related issue(s)

#1886

Checklist

• Automated checks pass
• Check if you need to add a changelog message N/A, augur subsample unreleased
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.07%. Comparing base (77f231d) to head (651fd74).
⚠️ Report is 71 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1890   +/-   ##
=======================================
  Coverage   74.07%   74.07%           
=======================================
  Files          82       82           
  Lines        8963     8964    +1     
  Branches     1821     1821           
=======================================
+ Hits         6639     6640    +1     
  Misses       2019     2019           
  Partials      305      305           

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