Prevent LazyLoadingViolationException when using strict mode

[Senexis]

Currently when using the package in conjunction with strict models, editing a menu will fail due to a LazyLoadingViolationException since the linkable is not eager loaded with the MenuItems. This pull request fixes that, making the package compatible with strict mode as far as I can tell.

Although usually the use of $with is discouraged, it seems the MenuItem class heavily relies on the relationship anywhere it is used. In the future, this could be improved by introducing specific ->with() calls on the various queries, but this should go a long way and shouldn't be too egregious.

The following should now work in the app's service provider:

use Illuminate\Database\Eloquent\Model;

Model::shouldBeStrict();

[datlechin]

Thank you