feat: custom section allow section titles generated by other tools

[gortavoher]

Description

Add features to the library to allow:

• the reading and writing of custom sections (H3 level headings that are not standard keep-a-changelog headlings)
• the reading and writing of a comment placed at the end of a release section

Changes

• refactor(token.rs): add HeadingLevel enum and use it to track heading levels in extract_tokens function
• fix(changes.rs): keep formatting for custom section when parsing custom change kind
• feat(changes.rs): add support for custom section in is_empty method
• feat(changes.rs): add support for custom section in Display implementation
• feat(parser.rs): add support for post comment in parse_footer method
• feat(release.rs): add support for post comment in Release struct and Display implementation
• fix(Cargo.toml): add parse-comment feature to default features
• feat(parser.rs): add parse_comment method to Parser struct
• feat(token.rs): add Comment variant to TokenKind enum and implement Display for it
• feat(changelog): add support for custom sections in the CHANGELOG.md file
• fix(changes.rs): add support for custom sections in ChangeKind and Changes structures
• feat(Cargo.toml): add feature 'custom-section' to allow custom sections in the CHANGELOG.md file
• test(changelog.rs): add test for reading and writing git-cliff changelog
• feat(changelog.md): add unreleased section with features, bug fixes, refactor, testing, and miscellaneous tasks