Add command to create a new release section from the unreleased section

[MalteHerrmann]

Using clu release [VERSION] it should be possible to add the corresponding release header for all changes in the Unreleased section.

It should also be checked that this release does not yet exist, and that it is the next sensible release according to semantic versioning. Maybe this also needs to be reflected in the configuration? That these changes are e.g. requiring a major or minor or patch version? It shouldn't necessarily enforce this, but rather post a notification for the user and ask for confirmation.