Implement flags in %autorelease macro
    
    This implements the prerelease, snapinfo and extraver flags, according to:
    https://docs.fedoraproject.org/en-US/packaging-guidelines/Versioning/#_more_complex_versioning
    
    Fixes: #152
    
    Signed-off-by: David Kirwan <dkirwan@redhat.com>
    Signed-off-by: Nils Philippsen <nils@redhat.com>