Help command can be interchanged out by disabling it, compact formatting for commands

I've seen commands displayed both ways, so I'm not sure which is more "unix-y", but this allows the user to switch between a compact format and the current format for commands. The current format for commands looks like so:

```
$ chat
usage: chat [<flags>] <command> [<flags>] [<args> ...]

A command-line chat application.

Flags:
  --debug              enable debug mode
  --help               Show help.
  --server=127.0.0.1   server address

Commands:
  help <command>
    Show help for a command.

  post [<flags>] <channel>
    Post a message to a channel.

  register <nick> <name>
    Register a new user.
```

By setting the following parameters:

```
app.SetCompactUsage(true)
app.SetHelpCmd(false)
```

The new output from above will look like:

```
$ chat
usage: chat [<flags>] <command> [<flags>] [<args> ...]

A command-line chat application.

Flags:
  --debug              enable debug mode
  --help               Show help.
  --server=127.0.0.1   server address

Commands:
  post        Post a message to a channel.
  register    Register a new user.

```

Again, I've seen the usage done both ways so these little switches allow you to leverage both. You don't have to merge this, but I found it useful for my own project.
Maybe just call this `UsageOnError`.