Add option to include unreleased changes when merging

[miniscruff]

changie merge --include-unreleased=<header>

The idea here is to run the equivalent of a batch as the latest version at the top of the changelog. Including unreleased without any unreleased should continue as normal.

The value of the parameter is the version header so you can do:

changie merge --include-unreleased="## Coming Soon"

Originally posted by @sheldonhull in #488 (reply in thread)

[sheldonhull]

This is more intuitive. I like that.