Feature add parameter wp additional inline config

[nicolasbrechet]

Hello,

I've added a "wp_additional_inline_config" parameter to allow loading of extra configuration options in the wp-config.php file without specifying a template.
This should solve issue #38.

I have tested it with Hiera:

wordpress::wp_additional_inline_config: 
  WP_CACHE: true

Which results in:

[...]

define('WP_CACHE', true);

/* That's all, stop editing! Happy blogging. */

Let me know if there's anything I should modify.

Thanks!