Modify completion as Result type

[cozzin]

Now we don't have to unwrapping OpenGraph.

OpenGraph.fetch(url: url) { result in
    switch result {
    case .success(let og):
        print(og[.title]) // => og:title of the web site
        print(og[.type])  // => og:type of the web site
        print(og[.image]) // => og:image of the web site
        print(og[.url])   // => og:url of the web site
    case .failure(let error):
        print(error)
    }
}

[satoshi-takano]

@cozzin It looks great! I'm grateful to your contribution. 🙏 Thanks.