record option could avoid writing null/default values

For example I obtained this kind of output where `no8cache`, `auth` and `size_hint` are not necessary.
Even `layout` can probably be avoided.

```yaml
  location:
    github_user: bucketfulofsunshine
    repository: baldurian-bar-hop
    tag: V0.91
    no_cache: false
    auth: null
    size_hint: null
    layout:
      layout_type: single_dir
```