Private data promotion function

This will render data like this:
```
- name: mbland
  full_name: Mike Bland
  private:
    email: michael.bland@gsa.gov
    slack: mikebland

- private:
  - name: foobar
    full_name: Foo Bar
    email: foo.bar@gsa.gov
    slack: foobar 
```
to this:
```
- name: mbland
  full_name: Mike Bland
  email: michael.bland@gsa.gov
  slack: mikebland

- name: foobar
  full_name: Foo Bar
  email: foo.bar@gsa.gov
  slack: foobar
```

@kaitlin @afeld, if you please! The next step will be cutting out code from `joiner.rb` and replacing it with calls to `JoinerImpl.remove_private_data` and `JoinerImpl.promote_private_data` as we switch to the new consolidated data format.