Add support to customize Parallels Vagrantfile

- convert key from snake_case to dash-case (prevents ugly yaml using
  quoted keys)
- the customize hash below should be considered as a default to allow one to
  close the vm window if Parallels Desktop is open:

```yaml
- name: parallels-vm
  driver:
    provider: parallels
    box: parallels/centos-65
    customize:
      on_window_close: keep-running
```

converts to:

```ruby
  c.vm.provider :parallels do |p|
    p.customize ["set", :id, "--on-window-close", "keep-running"]
  end
```
Merge pull request #1 from jhx/add_support_to_customize_parallels_vagrantfile

Add support to customize Parallels Vagrantfile