feat: improves plain output with audit confidence

[ubiratansoares]

Related to #19

Self-explained. This is a super simple proposal, eventually we can improve by implementing Display for Confidence, adding colors, etc.

Testing this PR :

• Pull this branch
• Run against some Workflow with existing audits

cargo run -- tests/test-data/hardcoded-credentials.yml

🌈 completed hardcoded-credentials.yml
error[hardcoded-container-credentials]: hardcoded credential in GitHub Actions container configurations
  --> tests/test-data/hardcoded-credentials.yml:15:7
   |
15 |         credentials:
   |  _______^
16 | |         username: user
17 | |         password: hackme
   | |________________________^ container registry password is hard-coded
   |
   = note: audit confidence → High

[woodruffw]

Great work, thanks @ubiratansoares!