Support "Keep a Changelog" format?

[job13er]

Should we update (or allow a configuration change to enable) supporting the Changelog format suggested in Keep a Changelog?

The main differences between their suggestions and how pr-bumper currently handles things:

1. Using ## instead of # for each release section
2. Allowing for an arbitrary top-level # and description above version sections (this means we have to be a little smarter than just pre-pending to the file).

The rest of the conventions have to do with the actual contents of the changelog section for the version. Those could possibly be validated by pr-bumper but I'd leave that as a potential, separate feature, as it would require quite a bit more effort and configuration in .pr-bumper.json likely.

[notmessenger]

I will need to dig into the specifics of it to see if there is something that "sticks out" but in general I am all for this. I have not used this specific format before but have done similar in other projects I maintain (https://github.com/softlayer/sl-ember-components/blob/master/CHANGELOG.md#0110) in order to provide a better user experience to consumers and even yourself often enough.

I will likely circle back around and add more content to this thread but wanted to give my initial 👍 on the idea.