Added allow to create related resource

[dom1no]

Resolve issue #28

A new method has been added:

• Method creatable(), This method allows you to create related resource

    // simple
     BelongsToManyField::make('Participants', 'participant', 'App\Nova\Participant')
     ->creatable(),

    // with settings
     BelongsToManyField::make('Participants', 'participant', 'App\Nova\Participant')
     ->creatable('Create new participant', ['additional_participant_field' => 'value']),

Used tagging functionality: https://vue-multiselect.js.org/#sub-tagging

[Benjacho]

Looks great, in what scenario do you need this feature?

[infostreams]

I created a fork just to get this functionality. I have the same use case (needing to create a new related resource). Would advise to merge.