extra config injected from ~/.docker-osx/Vagrantfile.extra_config

Adding a file name `Vagrantfile_extra.rb` into the `~/.docker-osx` directory will allow to keep some additional config for Vagrant without messing up too much the installation.

I used it for port forwarding some extra ports.

My `Vagrantfile_extra.rb` looks like:

```
config.vm.network :forwarded_port, guest: 3000, host: 3000
config.vm.network :forwarded_port, guest: 8080, host: 8080
````

This dirty patch doesn't want to be a final solution, just an idea to open the discussion about extend and customisation methods.