[Feature Request] Add "In Progress" section to CHANGELOG.md

[joeyparis]

I'm not sure how many people would actually use this feature but it would be great if we could have an optional "In Progress" section at the top of the changelog that included all of the "unreleased" changes. Our use case for this is when we are testing new features on stage before release it's helpful to have a list of the "unreleased" changes that we're currently testing.

An alternative solution would be to create a separate CHANGELOG_PREVIEW.md file that could list the unreleased changes for more organized testing purposes.

Feel free to close this if it's an undesired feature!