Add Store Subset of Registers

[craigthomas]

This PR adds the ability to store a subset of registers into memory starting from the index pointer. Note that the index register is not updated. Unit and integration tests added to cover new conditions. This PR closes #25

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.66%. Comparing base (1da3918) to head (02d4b61).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   90.27%   90.66%   +0.39%     
==========================================
  Files           4        4              
  Lines         473      493      +20     
==========================================
+ Hits          427      447      +20     
  Misses         46       46              

Files	Coverage Δ
chip8/cpu.py	97.46% <100.00%> (+0.13%)	⬆️

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