ENH: Add rocketry.args.TerminationFlag

[Miksus]

No description provided.

[codecov-commenter]

Codecov Report

Merging #49 (717b4c4) into master (05e205c) will increase coverage by 0.02%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   90.41%   90.43%   +0.02%     
==========================================
  Files          86       86              
  Lines        3860     3869       +9     
==========================================
+ Hits         3490     3499       +9     
  Misses        370      370              

Impacted Files	Coverage Δ
rocketry/args/builtin.py	92.18% <87.50%> (+0.95%)	⬆️
rocketry/args/__init__.py	100.00% <100.00%> (ø)
rocketry/core/task.py	91.51% <100.00%> (+0.03%)	⬆️

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