Implement "list" output format

[handrews]

Fixes #51

[codecov-commenter]

Codecov Report

Patch coverage: 83.33% and project coverage change: -0.03 ⚠️

Comparison is base (f0a4e16) 90.98% compared to head (ecf73b8) 90.96%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   90.98%   90.96%   -0.03%     
==========================================
  Files          21       21              
  Lines        1986     1992       +6     
  Branches      422      378      -44     
==========================================
+ Hits         1807     1812       +5     
  Misses        118      118              
- Partials       61       62       +1     

Impacted Files	Coverage Δ
jschon/output.py	95.34% <83.33%> (-0.91%)	⬇️

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[handrews]

This is not ready - I'd found the old branch and it passed the test and for some reason I thought that meant it was done but it definitely is not.