Passing multiple strategies to passport.authenticate()
Passport seems to [support multiple strategies being passed in an array to `passport.authenticate()`](https://github.com/passport/express-3.x-http-basic-and-digest-example/blob/master/server.js#L55)

In trying to get this to work with `koa-passport` though, I've been unable to get subsequent strategies tried after the first fails. I've tried debugging it, and it appears that `koa-passport` largely implements its own logic for how `authenticate` flows and makes no attempt to support multiple strategies. Is this an accurate assessment or am I just doing it wrong?