Add SubcommandChooser

[yegle]

The SubcommandChooser can be used to customize the behavior when the
default map lookup cannot find the subcommand you want.

Example usage:

busybox-style subcommand invocation

// A symbolic link `ls` to binary `busybox` will invoke `busybox ls`
mySubcommandChooser = func(*c cli.CLI) (CommandFactory, error){
    subcommand = os.Args[0]
    if subcommand != c.Name() {
        if subcommandFunc, ok := c.Commands[subcommand]; ok {
            return subcommandFunc, nil
        }
    }
    return cli.DefaultSubcommandChooser(c)
}

c := cli.NewCLI("busybox", "1.0.0")
c.Args = os.Args[1:]
c.Commands = map[string]cli.CommandFactory{
    "ls": lsCommandFactory,
    "cat": catCommandFactory,
}
c.SubcommandChooser = mySubcommandChooser

default subcommand

// `myapp --help` == `myapp default --help`
mySubcommandChooser = func(*c cli.CLI) (CommandFactory, error) {
    if c.Subcommand() == "" {
        return c.Commands["default"], nil
    }
    return cli.DefaultSubcommandChooser(c)
}
c := cli.NewCLI("myapp", "1.0.0")
c.Args = os.Args[1:]
c.Commands = map[string]cli.CommandFactory{
    "default": defaultCommandFactory,
}
c.SubcommandChooser = mySubcommandChooser

[yegle]

Did several update, now contains improved unit test coverage. PTAL.

[yegle]

ping?