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 `MultiCommand`s, 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. Testing Done: Used this extensively with the upcoming review command.