Adding -s=strict for using only the SPDX notation.

[tfroseman]

This preserves existing functionality by expanding valid values for the -s flag. "strict" may not be the most appropriate, but seemed suitable. This address Issue #195

New options are

-s
-s=only
-s=strict

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.19%. Comparing base (b289835) to head (10f837f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
main.go	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   49.80%   50.19%   +0.38%     
==========================================
  Files           2        2              
  Lines         259      261       +2     
==========================================
+ Hits          129      131       +2     
  Misses        123      123              
  Partials        7        7              

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