Improve suppressions handling

[ccojocar]

• Ensure the ignores are parsed before analysing the package
• Update README with details which describe the current behaviour of #nosec

fixes #1046

[codecov-commenter]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (7846db0) 72.30% compared to head (e89b7b1) 72.07%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
- Coverage   72.30%   72.07%   -0.23%     
==========================================
  Files          50       50              
  Lines        3394     3438      +44     
==========================================
+ Hits         2454     2478      +24     
- Misses        868      884      +16     
- Partials       72       76       +4     

Files	Coverage Δ
analyzer.go	84.23% <73.13%> (-3.27%)	⬇️

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