Default the interpreter to set memory size to 64K.

[craigthomas]

This PR changes the emulator to use a 64K memory space by default instead of the 4K memory space as defined by the original Chip8 language specification. This memory size can be changed by using the --mem_space flag if desired. Unit test added to catch new condition. This PR closes #24

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.08%. Comparing base (a0f7a93) to head (52925f3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   90.10%   90.08%   -0.03%     
==========================================
  Files           4        4              
  Lines         465      464       -1     
==========================================
- Hits          419      418       -1     
  Misses         46       46              

Files	Coverage Δ
chip8/cpu.py	97.26% <ø> (-0.01%)	⬇️

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