Allow own Model by configuration

[Nowi5]

Similar to Spatie/Analytics I would like to allow to set the Model by the config file. This provides the opportunity that the developer can extend the given model and enhance it. I have done this to improve performance by Model cache.

My config file:

    /*
     * This model will be used to maintain translations. The only requirement is that
     * it should be or extend the Barryvdh\TranslationManager\Models\Translation model.
     */
    'translation_model' => Barryvdh\TranslationManager\Models\Translation::class,

My Model:

<?php 
namespace  App\Models\Barryvdh;

use Illuminate\Database\Eloquent\Model;
use GeneaLabs\LaravelModelCaching\Traits\Cachable;
use Barryvdh\TranslationManager\Models\Translation as TranslationModel;

class Translation extends TranslationModel{

    use Cachable;

}

With that we can close #227

[isecchin]

I'm also using another package (hyn/multi-tenant) for a multi-tenant application, and have the tenant's connection set as the default, but the translation table is stored on the system's DB.

For that, the package provides a trait to use on the models to force a specific connection (system or tenant), but since the Translation Model cannot be overwritten, I can't apply the trait without forking the project.

I think this would be a very nice addition to the package !