refactor: code cleanup for the --negate flag

[nfischer]

No change to logic. This is a small refactor/cleanup for the '--negate' option added in PR #189.

This also adds documentation to the README.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6184003) to head (dab1982).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #233   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          116       116           
=========================================
  Hits           116       116           

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