Add option to Docker provisioner to build local images

Option would work like so:

```ruby
Vagrant.configure("2") do |config|
  config.vm.provision "docker",
    images: %w( ubuntu ),
    build_images: [
      { path: "Dockerfile", args: "-t me/my_app" }
    ]
end
```

Please be aware this is my first time hacking on Vagrant and I was kind of rushed : )

I didn't see any tests regarding this provisioner on mitchellh/vagrant#2549. Please let me know if I need to add something.

Also, is there documentation other than the `README` for hacking on Vagrant? I'm wondering what the normal work flow is for testing this feature I added. Would I just create a `Vagrantfile` with my new feature and run `bundle exec vagrant up`?

Thanks!