Error out if other flags are specified with --list or --help

[brettcannon]

Fixes #80

[codecov]

Codecov Report

Merging #81 (66db6a5) into main (1f97216) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #81   +/-   ##
=======================================
  Coverage   99.69%   99.70%           
=======================================
  Files           2        2           
  Lines         328      336    +8     
=======================================
+ Hits          327      335    +8     
  Misses          1        1           

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/cli.rs	99.37% <100.00%> (+0.03%)	⬆️

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