feat: add changelog modifier callback to run function

[weichweich]

Description

I added a call back function to manipulate the Changelog before it's written or the context is printed (depending how git-cliff is started).

Motivation and Context

This change is really handy to add custom logic to query information from GitLab etc. I would love to have something similar to plug-ins, but adding this call back is trivial and allows to build custom git-cliff versions that execute use-case specific, custom logic.

How Has This Been Tested?

This should be trivial. What could possibly break!? :sweat_smile:

Screenshots / Logs (if applicable)

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (no code change)
• [ ] Refactor (refactoring production code)
• [ ] Other

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly.
• [x] I have formatted the code with rustfmt.
• [x] I checked the lints with clippy.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 42.12%. Comparing base (bbb805d) to head (030de7b).

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #922      +/-   ##
==========================================
- Coverage   42.70%   42.12%   -0.57%     
==========================================
  Files          21       21              
  Lines        1745     1769      +24     
==========================================
  Hits          745      745              
- Misses       1000     1024      +24     

Flag	Coverage Δ
unit-tests	42.12% <0.00%> (-0.57%)	:arrow_down:

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[orhun]

Hey, really interesting change! I'm down to add it, but maybe add some documentation or examples on how to use this from the API side. Thanks for the PR!