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
```