Update release notes parsing

[spencertransier]

What does it do?

This PR adds more parsing options to the release_notes_helper.rb so that Markdown headers and lists can be used at the top of the release notes/changelog files.

@mokagio suggested this change so that a client could use this kind of formatting in their release notes file:

# Release Notes

## Importance Indicators
- Use `[+++]` to call out major new features.
- Use `[++]` to describe other interesting improvements
- Use `[+]` to describe minor improvements or bugfixes.

## Platforms
- Please add `iOS`, `Mac`, or `Both` to indicate which platform the change affects
- Example:
- [+++] Mac: Added a new photo picker

This remains backwards-compatible with the existing client release note formatting.

Checklist before requesting a review

• [X] Run bundle exec rubocop to test for code style violations and recommendations
• [X] Add Unit Tests (aka specs/*_spec.rb) if applicable
• [X] Run bundle exec rspec to run the whole test suite and ensure all your tests pass
• [x] Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• [X] If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider.

[crazytonyli]

@spencertransier The two new "build-and-test" required PR checks are added from https://github.com/wordpress-mobile/release-toolkit/pull/492. Merging the trunk branch into this PR should satisify those two new required checks.

[spencertransier]

@crazytonyli Thank you!