[WIP] Document settings through "behat --config-reference"

As my learning curve still is steep it would be great to have the config documented and learn people to use

```bash
behat --config-reference
```

- [ ] document all other options

Please comment on all settings so I can work on their documentation.

```bash
$ vendor/bin/behat --config-reference
...
    drupal:
        basic_auth:
            username:             ~
            password:             ~

        # Use "blackbox" to test remote site. See "api_driver" for easier integration.
        default_driver:       blackbox

        # Bootstraps drupal through "drupal8" or "drush".
        api_driver:           drush
        drush_driver:         drush
        region_map:

            # Prototype
            key:                  ~
        text:
            log_in:               'Log in'
            log_out:              'Log out'
            password_field:       Password
            username_field:       Username
        selectors:
            message_selector:     ~
            error_message_selector:  ~
            success_message_selector:  ~
            warning_message_selector:  ~
        blackbox:             []
        drupal:
            drupal_root:          ~
        drush:
            alias:                ~
            binary:               drush
            root:                 ~
        subcontexts:
            paths:

                # Prototype
                key:                  ~
            autoload:             true