feat(changelog)!: support templating in the footer

[orhun]

Description

This PR makes it possible to use a template for the footer.

Before:

# changelog footer
footer = """
<!-- generated by git-cliff -->
"""

After:

# template for the changelog footer
footer = """
<!-- generated by git-cliff at {{ now() | date(format="%Y-%m-%d") }} -->
"""

Motivation and Context

Now we can have links at the footer for having the exact format as Keep a Changelog specification:

# template for the changelog footer
footer = """
{% for release in releases %}\
    {% if release.version %}\
        {% if release.previous.version %}\
            [{{ release.version | trim_start_matches(pat="v") }}]: <REPO>/compare/{{ release.previous.version }}..{{ release.version }}
        {% endif %}\
    {% else %}\
        [unreleased]: <REPO>/compare/{{ release.previous.version }}..HEAD
    {% endif %}\
{% endfor %}
<!-- generated by git-cliff -->
"""

Closes #95

How Has This Been Tested?

• Unit tests
• Integration tests
• 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: 9 lines in your changes are missing coverage. Please review.

Comparison is base (1e571c2) 43.40% compared to head (615ee6a) 43.76%.

Files	Patch %	Lines
git-cliff-core/src/changelog.rs	61.12%	7 Missing ⚠️
git-cliff-core/src/release.rs	0.00%	1 Missing ⚠️
git-cliff-core/src/template.rs	90.91%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #369      +/-   ##
==========================================
+ Coverage   43.40%   43.76%   +0.37%     
==========================================
  Files          13       13              
  Lines         643      649       +6     
==========================================
+ Hits          279      284       +5     
- Misses        364      365       +1     

Flag	Coverage Δ
unit-tests	43.76% <70.00%> (+0.37%)	⬆️

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