Customize screens

Replaces showsVideo by more versatile screens api.

public var screens: [YPPickerScreen] = [.library, .photo]

Usage

...
config.screens =  [.library, .photo, .video] // show video as well

...
config.screens =  [.photo] // .photo only

Shout out to @ChintanWeapp & @eddieespinal for initiating the idea 👏