Add command option to specify the source files to run the coverage on (#806)

[cgewecke]

#805 (Continuation of PR #806)

Have

• added the ability to specify a single source
• made the argument relative to paths.sources (this differs from the the --testfiles works but is the same as skipFiles)
• documented the option in the README

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8da877e) 96.49% compared to head (c5c2268) 96.51%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #838      +/-   ##
==========================================
+ Coverage   96.49%   96.51%   +0.01%     
==========================================
  Files          19       19              
  Lines        1057     1061       +4     
==========================================
+ Hits         1020     1024       +4     
  Misses         37       37              

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