Automatically populate changelog/tag message

[amyreese]

Description

Currently, when running attribution tag, the tag message template will include a list of commits since the previous tag, but the actual message will be empty. Many projects will keep an ongoing record of release notes in their CHANGELOG.md file leading up to a release, often at the top under a section heading named "next", "upcoming", or whatever the next expected release name will be.

It would be nice for attribution to look at the changelog itself, before generating the tag message template, to look for these common heading names or patterns, and if they exist, copy the contents of that section into the message template. For projects that are diligent about updating the changelog ahead of time, this will make new releases much easier, or at least remove the need to copy/paste the relevant portion of the changelog into the commit message.

For now, this could be done with some basic regex or markdown parsing, but with the potential for configurable changelog templates in the future, using a configurable search mechanism would be useful for allowing users to better specify what attribution should be looking for. The easy way is good enough for now if the configurable method doesn't fit well in the existing workflow.