We'll soon be adding two new commands, rbt api and rbt review, which

both want to implement additional subcommands (rbt api get,

rbt review publish, etc). This change adds the framework to do so,

consisting of two new base classes. SubCommand should be inherited for

each subcommand offered, and MultiCommand aggregates them together.

Arguments from the MultiCommand are added in to the subcommand

parsers.
The one tricky thing here was getting --help to do the right thing. In

the case of MultiCommands, we need to actually parse the arguments in

order to get the right subparser, but doing so was enforcing any usage

requirements. It turns out that the correct fix here was to allow

add_help to default to True in the parser initialization. This

allows us to call parse_args and have it not fail.
I did also have to move the logging setup out of the initialize

method, so that we wouldn't attach multiple log handlers.

Used this extensively with the upcoming review command.