nextToken/prevToken for future records

[andreialecu]

Hey, I was looking into this library and was wondering if the following could be supported:

nextToken should be a valid token even if there is no next page. Assume a webpage got a list of messages, but then via websocket it is notified of new data to fetch.

It should be possible to use the cursor to get results that were added in the mean time.

It should be possible to have both a nextToken and a prevToken.

So, as an example, a chat app would work like this:

1. get 50 latest chat messages (sorted by date: -1)
2. there should be a way to get "future" messages, once an event comes via websocket
3. should be also possible to get previous messages, when the user scrolls back and wants to see the chat room history.

In this case, if sorted by date: -1, the nextToken would actually be for old messages, and prevToken would be for future messages.