Add support for Drupal 8 configuration

Currently, this module writes Drupal 7 configuration, but not Drupal 8. To use the module effectively on Drupal 8 sites, we'll need to convert lines like:

$conf['site_name'] = 'My Drupal site';

...to:

$config['system.site']['name'] = 'My Drupal site';

It's only the part before the equal sign that changes. So we should do the following:

1. Use config instead of conf.
2. Convert all instances of a new designated delimiter to ']['. I think ~ is a good choice because it's in the Unreserved Characters list.
3. Update the documentation on the site config form, Variables, to explain use of the delimiter for Drupal 8 and include it in the examples.