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 = '_';
    ...
}
```
So do [a zillion other places in the October code](https://github.com/octobercms/library/blob/master/src/Database/Model.php#L234).