feat(config): allow configuring output file from config

[orhun]

Description

This PR adds a new configuration option as follows:

[changelog]
output = "CHANGELOG.md"

It does not take precedence over command-line arguments which means you can override it with the --output option.

Motivation and Context

closes #827

How Has This Been Tested?

Locally.

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]

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Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 40.10%. Comparing base (c34aaa0) to head (1da10e9).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #829      +/-   ##
==========================================
- Coverage   40.19%   40.10%   -0.08%     
==========================================
  Files          20       20              
  Lines        1645     1646       +1     
==========================================
- Hits          661      660       -1     
- Misses        984      986       +2     

Flag	Coverage Δ
unit-tests	40.10% <0.00%> (-0.08%)	⬇️

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