It enables to have following statements in `_config.yml`:

```yaml
jekyll_get:
  data: attendees
  json: https://event.api/attendees?event_id=148916&api_key=<%= ENV['API_KEY'] %>
  cache: true
```

`API_KEY=foobar bundle exec jekyll serve` will be interpolate at runtime as:

```yaml
jekyll_get:
  data: attendees
  json: https://event.api/attendees?event_id=148916&api_key=foobar
  cache: true
```

I use `SafeYAML` because this is how jekyll parses its YAML files and because we are the owners of the config files, I guess it is safe enough to use ERB to interpolate Ruby code only and specifically in the `jekyll_get` section.