braindump: rerenderables

Nobody has claimed this yet.

@jimmyhchan talked about this last year and I think it finally clicked for me.

`dust.render` returns undefined right now and uses a callback. Jimmy talked about "streaming into templates" last year and I never quite grokked it, but I think this might be it.

What about an API something like this:

```
let renderable = dust.render(template, context, function(err, out) {
// some side effect, appending to or replacing DOM or whatever
});

renderable.render(anotherContext); // uses the same template and the same callback
someStream.pipe(renderable); // maybe this invokes the callback once for every data event
```

This would give you a `renderable` that you could store and reuse to update a certain portion of DOM with new data. Identifying when data changed would still be your responsibility, this is just so you don't have to manually wire it all up again.

Start by reading the existing dust.render API and its callback behavior. Clarify the reusable renderable and stream semantics before implementation, including how repeated renders invoke the callback. Done should include an agreed API and tests covering the proposed usage.

Written by the indexing model from the issue text.