Format extracted_license_statement as YAML

[AyanSinhaMahapatra]

See aboutcode-org/scancode.io#715 (comment)

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Looked for possible updates in documentation and added updates if applicable
• Updated CHANGELOG.rst

[AyanSinhaMahapatra]

@pombredanne this is ready to review! We can do a rc5 after this is merged.

Btw, this was already in the changelog from this commit: d264aec but it was not YAML yet

[pombredanne]

Thanks! we still need a few mini tests IMHO for a YAML, JSON and JSON-line outputs in formattedcode where we would have a few packages with such statements. I suggest an old npm (with a mapping), a new npm (with a single string as an expression) a Maven and PyPI with classifiers and license to have sample of the multiple use cases.

[pombredanne]

LGTM, we can add other output tests later

[AyanSinhaMahapatra]

@pombredanne thanks, I've also added the tests. Merging!