Add ability to define subcommands in the same file

[frankcarey]

Right now, you have to import another file to have subcommands, but it would be nice to:

1. Declare subcommands in the parent file itself, making it easier to have full control over the cli app.

2. Override subcommands from the parent, so you can override a subset of commands in another file without copying the entire file. (also useful if you have a standard set of commands across projects, but you only need to override one of them)

[dkinzer]

+1

[bluengreen]

+1