Push cmds.Cmd._add_subcommands() as lib.Cmd.load_subcommands() one
step up to the top of the type hierarchy ladder.
By default, it does the same thing, i.e. load subcommands matching
frobnicate.Cmd* if called on class CmdFrobnicate.
This commit also replaces invocations of Cmd._add_subcommands() by
invocations of this new method.
Signed-off-by: Jan Lindemann <jan@janware.com>