Feature add parameter wp additional inline config

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!