Add slugsSeparator option

This PR adds an option to set the separator used in the Sluggable trait. Defaults to `-`

## Usage

```php
class Category extends Model
{
    use \October\Rain\Database\Traits\Sluggable;

    public $slugs = ['slug' => 'title'];
    public $slugsSeparator = '_';
    ...
}
```
This breaks PSR-2 5.1