feat(changelog): support bumping the semantic version via --bump

[orhun]

Description

git-cliff will automatically bump the version for your unreleased changes in the changelog.

For example, if you have 1.0.0 and committed "feat: xyz", git-cliff --bump --unreleased will create a changelog for 1.1.0.

Motivation and Context

Closes #163

How Has This Been Tested?

Unit tests.

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: 12 lines in your changes are missing coverage. Please review.

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Files	Coverage Δ
git-cliff/src/args.rs	100.00% <ø> (ø)
git-cliff-core/src/release.rs	53.85% <63.64%> (+53.85%)	⬆️
git-cliff/src/lib.rs	0.00% <0.00%> (ø)

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