Refactor changelog entry generation
    
    Previously, a changelog entry was pieced together directly in the
    changelog visitor and a slightly tweaked copy of that code existed in
    run() which processed uncommitted changes.
    
    This moved changelog generation into its own module containing
    ChangelogEntry, an augmented special purpose dictionary. It carries all
    information necessary to generate a changelog entry and contains the
    logic in its format() method.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>