feat(changelog): improve skipping via .cliffignore and --skip-commit

[orhun]

Description

This PR improves the skipping commits functionality.

1. .cliffignore

You can now add a .cliffignore file at the root of your repository and list the commits that will be skipped:

# skip commits by their SHA1

4f88dda8c746173ea59f920b7579b7f6c74bd6c8
10c3194381f2cc4f93eb97404369568882ed8677

2. --skip-commit

You can skip commits by using this argument as follows:

$ git cliff --skip-commit 10c3194381f2cc4f93eb97404369568882ed8677 \
            --skip-commit 4f88dda8c746173ea59f920b7579b7f6c74bd6c8

Motivation and Context

Closes #390

How Has This Been Tested?

Test fixtures.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• 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:

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

[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (0ca4cdb) 42.42% compared to head (bc60467) 41.75%.

Files	Patch %	Lines
git-cliff/src/lib.rs	0.00%	16 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #413      +/-   ##
==========================================
- Coverage   42.42%   41.75%   -0.67%     
==========================================
  Files          15       15              
  Lines         995     1011      +16     
==========================================
  Hits          422      422              
- Misses        573      589      +16     

Flag	Coverage Δ
unit-tests	41.75% <0.00%> (-0.67%)	⬇️

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