Add command line color specification

[craigthomas]

This PR adds the ability to specify what colors should be used on the command line. There are 4 new switches added:

• color_0 for the background / clear color.
• color_1 for the first bitplane color.
• color_2 for the second bitplane color.
• color_3 for the bitplane overlap color.

Default values are specified to sensible values. This PR closes #30

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.92%. Comparing base (1da3918) to head (80365d7).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   90.27%   90.92%   +0.65%     
==========================================
  Files           4        4              
  Lines         473      507      +34     
==========================================
+ Hits          427      461      +34     
  Misses         46       46              

Files	Coverage Δ
chip8/emulator.py	0.00% <0.00%> (ø)
chip8/screen.py	90.32% <85.71%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b9e1be7...80365d7. Read the comment docs.