Have one way and only one way to set command group

[strugee]

It's very hard to figure out how to set the command group from the code. Most commands do it explicitly, in start(). But !tea doesn't, and that happens to be the one I looked at for examples (because it's the simplest). !tea relies on this implicit default.

One of these ways should be killed. I think also in general it should probably be way more obvious how to add a new command... I'm struggling.