Allow END macro to set exec address

[craigthomas]

This PR updates the END macro so that it takes an optional label. The address of the label is then stored as the exec address of the assembly language program when saved as a CAS or DSK file. Unit tests updated to catch new conditions. This PR closes #99

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.92%. Comparing base (0319eb7) to head (d3c0cdf).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   94.90%   94.92%   +0.02%     
==========================================
  Files          16       16              
  Lines        2041     2050       +9     
==========================================
+ Hits         1937     1946       +9     
  Misses        104      104              

Files with missing lines	Coverage Δ
cocoasm/program.py	99.37% <100.00%> (+0.03%)	⬆️

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