Support generating changelog section for Unreleased commits

[mholttech]

Found this repo today after spending the weekend looking at standard-version and longing for parserOpts & writerOpts capabilities. There's one more capability i'm looking for that I can't seem to find so far.

conventional-changelog-core supports a configuration option to output unreleased commits to to the changelog. I would love to see this ability added.

See:
https://github.com/conventional-changelog/conventional-changelog/tree/master/packages/conventional-changelog-core#outputunreleased

[TimothyJones]

Seems like a reasonable addition to me, and it would be inline with the fork rationale.

If you (or anyone reading this) has the time to make the change, I'd happily accept any PRs that added this.