Add command blacklist support to bundler plugin

The user can set a `$UNBUNDLED_COMMAND` option in their .zshrc:

```bash
$UNBUNDLED_COMMAND=foreman
# or
$UNBUNDLED_COMMAND=(cucumber foreman rake)
```

The bundler plugin for oh-my-zsh won't force these commands to run with `bundle exec`.

It's much easier to exclude some commands like this than having to fork the whole project.