Retrieve multiple user streams

[scruffyfox]

I think it would be cool if you could retrieve multiple users' streams by using the comma seperated idiom similar to the other requests, this could be used on the client side to create a 'list' type service.

E.G. https://alpha-api.app.net/stream/0/users/16673,1/posts

[scruffyfox]

The way we could unify lists cross-application would be the same way @ryantharp does with list-app.net, by taking advantage of channels and storing the user ids in that list of the streams to fetch. This would work so well because it

1. already exists in the API
2. allows for public/private lists
3. not much work needed to be done on the client side if it already supports channels
4. cross application syncing

Also bump