wip: the Command doc
remove: basecmd parameter to Command.run()

It is not used for anything except the hack in list/info which there are handful
of ways of handling nicely.
doc: api: dnf.cli. Commands and resgistering them.

This is essential for plugin authors who wish to register their own commands.
remove: Command.needTs*(). Unused.
api: plugins: transaction() hook called after a transaction. (RhBug:967264)
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
tests: test drpm to rpm failover
Hide the progress bar on failfast abort