Optional done button disabled state

[bcylin]

Add an optional delegate method to toggle the done bar button item based on the selected assets:

protocol ImagePickerControllerDelegate {

  /// Optional. Asks the delegate for the flag to enable the done bar button item based on the selected assets. The default behaviour `isEnabled = !assets.isEmpty`.
  @objc optional func imagePickerController(_ picker: ImagePickerController, shouldEnableDoneBarButtonItemWithSelected assets: [PHAsset]) -> Bool

}

Close #38.

[carouselljenkins]

@HungnguyenVN, can you review this pull request?

[amelia2801]

lgtm