add --parallel option

[epi052]

closes #213

Branching checklist

• There is an issue associated with your PR (bug, feature, etc.. if not, create one)
• Your PR description references the associated issue (i.e. fixes #123456)
• Code is in its own branch
• Branch name is related to the PR contents
• PR targets master

Static analysis checks

• All rust files are formatted using cargo fmt
• All clippy checks pass when running cargo clippy --all-targets --all-features -- -D warnings -A clippy::deref_addrof -A clippy::mutex-atomic
• All existing tests pass

Documentation

• New code is documented using doc comments
• Documentation about your PR is included in the README, as needed

Additional Tests

• New code is unit tested
• New code is integration tested, as needed
• New tests pass

[codecov]

Codecov Report

Merging #222 (fc35bb6) into main (9e0118f) will increase coverage by 0.03%.
The diff coverage is 97.61%.

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
+ Coverage   95.01%   95.05%   +0.03%     
==========================================
  Files          58       58              
  Lines        5763     5805      +42     
==========================================
+ Hits         5476     5518      +42     
  Misses        287      287              

Impacted Files	Coverage Δ
src/scan_manager/tests.rs	97.15% <ø> (ø)
src/banner/container.rs	99.57% <75.00%> (-0.43%)	⬇️
src/config/container.rs	97.96% <100.00%> (+0.01%)	⬆️
src/config/tests.rs	99.57% <100.00%> (+<0.01%)	⬆️
src/main.rs	95.31% <100.00%> (+0.86%)	⬆️
src/parser.rs	100.00% <100.00%> (ø)
src/filters/init.rs	97.72% <0.00%> (+2.27%)	⬆️

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