Show usage of child commands for their parent in help text

[Tyrrrz]

For a command that has child commands sub1 and sub2 show the subcommands in usage.

Usage
  myapp command [options]
  myapp command sub1 [options]
  myapp command sub2 [options]

Probably best to implement this after #10

[Tyrrrz]

The idea here is to make it easier for the user to tell how the subcommands work without having to query help on a subcommand.

Example (it doesn't actually do that):

> docker container --help

Usage
   docker container [options]
   docker container ls [options]
   docker container kill <id> [options]

[ADustyOldMuffin]

I'd like to also take this one on.

[Tyrrrz]

Go ahead.