Configure output streams to support non-Unicode encodings

[victorlin]

Description of proposed changes

Fixes a bug where Unicode characters in Augur's help output would result in an unhelpful error. This isn't expected to happen for most users. At the expense of those users, keep using Unicode characters; for them, print replacement characters.

Related issue(s)

Fixes #1272

Testing

• PYTHONIOENCODING=latin-1 augur clades --help runs successfully with these changes.

Checklist

• Add a message in CHANGES.md summarizing the changes in this PR that are end user focused. Keep headers and formatting consistent with the rest of the file.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (b00afc1) 69.86% compared to head (fd85168) 69.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1290      +/-   ##
==========================================
+ Coverage   69.86%   69.87%   +0.01%     
==========================================
  Files          67       67              
  Lines        7155     7158       +3     
  Branches     1744     1744              
==========================================
+ Hits         4999     5002       +3     
  Misses       1849     1849              
  Partials      307      307              

Files Changed	Coverage Δ
augur/__main__.py	100.00% <100.00%> (ø)

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