Add an 'output_location' argument

Introducing an option to choose where the dynamic CSS and Google Fonts will be applied (as mentioned in #1590).

```php
'output_location' => array('frontend', 'login', 'admin'),
```

This allows to toggle outputting the CSS on the frontend, the login/register pages, and even the backend independently. The default value, set in set_default_args, is `array('frontend')` to make this option backward compatible.

Please let me know if you have any concerns about this approach.

*Also, my editor applied the provided .editorconfig configuration and thus remove quite a lot of trailing whitespace. If you add `?w=1` to the URL, you should be able to view only non-whitespace changes.*