Add _cmd_setup_dirs function for homebrew installation.

Homebrew can't create directories in $HOME and Homebrew shouldn't download any files, so I cannot run `setup` subcommand in Formula of Homebrew. Instead I want to run `setup_dirs` in Formula of Homebrew like below. (It just creates directories)

```
$ perl nodebrew setup_dirs
```

See more details https://github.com/Homebrew/homebrew/pull/29729#issuecomment-44752257