Added ability to inject custom converters with an event

[bvangennep]

• [x] I updated the changelog
• [x] I updated the composer.json version
• [ ] I wrote tests
• [x] I am proud of my code

Custom converters can be injected with the EVENT_RESOLVE_CONVERTER event. This can be especially useful for importing and exporting custom field types. The converters need to implement the NerdsAndCompany\Schematic\Interfaces\ConverterInterface.

Event::on(Schematic::class, Schematic::EVENT_RESOLVE_CONVERTER, function (ConverterEvent $event) {
    if ($event->modelClass = "My\Custom\Field") {
      $event->converterClass = "My\Custom\FieldConverter";
    }
});

Fixes #123