Support other frontmatter formats (TOML, JSON, custom delimiters)

[redtux]

Our SPDXMarkdown changelog format plugin currently only strips ----delimited YAML frontmatter before delegating to commitizen's built-in Markdown parser.

woile's comment on commitizen#2014 (the upstream issue that motivated this plugin) made this limitation clear:

Frontmatters come in different shapes, you are assuming --- and yaml, but those are not all the cases. We are not going to hardcode a solution that fits only your use case.

We fully agree — and that is exactly why this is a plugin rather than an upstream patch. But right now our implementation only handles one shape (--- YAML), leaving other common frontmatter formats unsupported:

• TOML frontmatter: delimited by +++
• JSON frontmatter: delimited by { } (though JSON is less common in practice)
• Other custom delimiters: projects may use arbitrary markers

How to help

The core logic lives in _strip_frontmatter (standalone, no commitizen dependency). Adding support for a new format means:

1. Extending _strip_frontmatter (or adding a sibling function) to handle the new delimiter
2. Creating or extending config knobs so users can select which frontmatter format to strip
3. Adding tests in tests/test_spdx_markdown.py
4. Updating docs

The architecture is deliberately designed with only two override points in _SPDXMarkdownImpl — get_metadata and get_latest_full_release — so the integration surface is tiny.

See the design doc for the full class hierarchy and algorithm.

PRs welcome!