Add Index Load Long

[craigthomas]

This PR adds the load long instruction for the index register. Given that the new load command consumes another 16-bit value from memory, all skip instructions where modified to check for a load long instruction so that they would be incremented by 4 bytes instead of 2. Unit tests updated and added to catch new condition. This PR closes #23

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.30%. Comparing base (1da3918) to head (8fd9816).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   90.27%   91.30%   +1.02%     
==========================================
  Files           4        4              
  Lines         473      529      +56     
==========================================
+ Hits          427      483      +56     
  Misses         46       46              

Files	Coverage Δ
chip8/cpu.py	97.67% <100.00%> (+0.34%)	⬆️

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