Add Bitplane Selection

[craigthomas]

This PR adds a bitplane register to the CPU, along with an associated operation to set the bitplane to draw on. Additionally, this PR moves a few of the subroutines that were originally encapsulated within an if statement into separate functions which are called through the use of dictionary lookups instead (all operations starting with 00). Unit tests added to catch new conditions. This PR closes #27

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.37%. Comparing base (1da3918) to head (87b989b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   90.27%   90.37%   +0.10%     
==========================================
  Files           4        4              
  Lines         473      478       +5     
==========================================
+ Hits          427      432       +5     
  Misses         46       46              

Files	Coverage Δ
chip8/cpu.py	97.36% <100.00%> (+0.03%)	⬆️

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