api: make Command available directly from dnf.cli.
rename: Cli._register_command() -> register_command().

The plugins call it so it should be public.
drpm: add config options + docs, load updateinfo.xml
get_package_target(): don't use yum compatibility attribute
drpm: download delta packages if possible
tests: add repository with delta rpms
tests: MockBase() should load real repos, too.
tests: test rpm and drpm downloads from repo with deltas
get_package_target(): support the "donecb" callback
drpm: spawn delta rebuild jobs, report failures
drpm: fall back to rpm download on drpm errors
Hide the progress bar on failfast abort