add structured log output

[epi052]

closes #124

Landing a Pull Request (PR)

Long form explanations of most of the items below can be found in the CONTRIBUTING guide.

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 [FEATURE REQUEST] Dynamically tuning concurrency/connection limit or bailing after n connection failures #123)
• 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
• 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 #133 (2be2da4) into master (3881789) will decrease coverage by 0.08%.
The diff coverage is 98.23%.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
- Coverage   93.79%   93.71%   -0.09%     
==========================================
  Files          13       13              
  Lines        2208     2179      -29     
==========================================
- Hits         2071     2042      -29     
  Misses        137      137              

Impacted Files	Coverage Δ
src/scan_manager.rs	88.47% <ø> (ø)
src/heuristics.rs	91.15% <92.30%> (-2.97%)	⬇️
src/config.rs	96.40% <98.21%> (+0.23%)	⬆️
src/banner.rs	99.70% <100.00%> (+<0.01%)	⬆️
src/logger.rs	100.00% <100.00%> (+2.94%)	⬆️
src/main.rs	90.68% <100.00%> (-0.06%)	⬇️
src/parser.rs	100.00% <100.00%> (ø)
src/reporter.rs	88.88% <100.00%> (-1.77%)	⬇️
src/scanner.rs	92.06% <100.00%> (ø)

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