let instagram: Provider = .instagram(clientID: "***", redirectURL: "foo://callback")
instagram.authorize { result in
print(result) // success(Token(accessToken: "abc123"))
} 提供者包括：
GitHub - doc Dribbble - doc Instagram - doc Uber - doc Feedly - doc Vimeo - doc SoundCloud - doc StackExchange - doc Medium - doc Foursquare - doc Stripe - doc Reddit - doc Weibo - doc Slack - doc Dropbox - doc Basecamp - doc Spotify - doc Meetup - doc Strava - doc Google - doc Stuart - doc UberRUSH - doc