feat(config): add changelog.always_render option

[orhun]

Description

This PR adds a new option to the configuration file as follows:

[changelog]
always_render = true

The reasoning behind this is that some users might want to always render the changelog, even if there are no changes.

Implemented as a config option to avoid breaking changes.

Motivation and Context

closes #802

How Has This Been Tested?

Test fixtures / 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.