Request: make "selectable" binding checked on changes

[conrado-l]

I think it would be a good idea to make the "selectable" input/prop bindable.

I'm making a feature and when there is a loader, I would set "selectable" to false until the loading finishes, I came to realize that the "selectable" input is being checked on ngOnInit, and because of it, it's not bindable.

I think every input or almost every input should be bindable for maximum flexbility.

Thanks for your time and mantaining this awesome library.

[AckerApple]

https://github.com/AckerApple/angular-file/blob/development/src/file-upload/ngf.directive.ts#L33

[AckerApple]

Oh I see, it only checks on init. Ok. Maybe make a pull request then please