Add "subcommand chooser"

[yegle]

The cli.CLI struct should add a SubcommandChooser, which is a func that return a cli.Command.

The reason would be when the specified subcommand doesn't exist (see also pull reqeust #10) the behaviour can be controlled by the user.

Also I can implement something similar to busybox's behaviour that by creating a link foo to my binary, I can invoke the foo subcommand directly.

If this idea looks good to you I can send a PR and implement it.

[mitchellh]

This looks good! If you submit a PR I'd be happy to merge this. Going to close this issue though since I don't plan on doing this myself in the short term. Thanks!

[yegle]

FYI this is implemented in #20