Allow editing the config options of single instance more easily

at this time, the SwapperView instance's config property is pointing to the default config:

public class SwapperView: UIView {
    /// Access to `SwapperViewConfig` to set defaults on all instances of `SwapperView`.
    public static let defaultConfig: SwapperViewConfig = SwapperViewConfig.shared
    /// Override `defaultConfig` for this once instance.
    public var config: SwapperViewConfig = SwapperViewConfig.shared {
        didSet {
            configView()
        }
    }

This does not allow you to call instance.config.changeSingleValue = 4. I need to be able to change 1 property of an instance without touching the default.

Start at the SwapperView class and its config and defaultConfig properties shown in the issue, then inspect SwapperViewConfig.shared and the configView() call. Determine how an instance can change one configuration value without modifying shared defaults; done means a single instance can edit a property independently while other instances and the default remain unchanged.