Added support for abort_on_nonzero and cwd configs

Hi, 

I was trying to use your plugin but I needed 2 features:
- Abort provisioning if the script exits with a non-zero code
- Defining a cwd for execution of the script.

I added this via configs, ie:
```
  config.vm.provision :host_shell do |host_shell|
    host_shell.cwd     = File.expand_path File.dirname(__FILE__)
    host_shell.inline  = "./some-script/data/script.sh"
    host_shell.abort_on_nonzero = true
  end
```

I hope you find this useful :)

Cheers,
Carlos