Adds new helper `content_settings_value`

This helper returns a value either from `content.settings` or from `options` hash.

What this brings?

You can now set essence view partial options in the `elements.yml` file:

```
- name: image
  contents:
  - name: image
    type: EssencePicture
    settings:
      image_size: '100x80'
```
Merge pull request #644 from magiclabs/feature/add_content_settings_value

Adds new helper `content_settings_value`