Swift

This adds support for generating Swift projects.

Yeah, I know, _finally_.
Maybe you could extract a `#build_config` helper that sets up all the needed data, so that your test can just be:

```ruby
it 'returns the swift app target groups' do
  config = build_config('swift')

  expect(config.app_target_groups).to eq(swift_groups)
end

def build_config(project_template)
  swift_groups = {'swift' => 'groups'}
  objc_groups = {'objc' => 'groups'}
  templates = {'swift' => swift_groups, 'objc' => objc_groups}

  Liftoff::ProjectConfiguration.new({
    :project_template => project_template,
    :app_target_templates => templates
  })
end
```