Reorganize subcommands

This doesn't actually work yet, but it's what I'd like the API to look like once wp-cli/wp-cli#642 is handled.

Advantages over the homegrown component solution:

* documentation is in the same file as the implementation
* subcommands are grouped logically by their component

So, for example, when you run `wp bp`, you'll get the list of components. When you run `wp bp group`, you'll get the group subcommands etc.