Add Support for custom Store Key

[mlch911]

In some cases, like inside an extension of a protocol, we can't add a static variable as the store key. This can provide custom store key by outside.

private var customKey = ""
extension Hashable {
    @AssociatedObject(.OBJC_ASSOCIATION_RETAIN_NONATOMIC, key: customKey)
    var text = ""
}

[p-x9]

I thought it would be useful to be able to use custom keys regardless of how the keys are implemented(#28), so I am considering merging them.