Allow filters to express implicit dependencies

```js
myFilter = {
  filter:    ... // The filter function.
  extraDeps: ... // A function that returns an array of observers.
};
```

An example of where I'm using it: https://github.com/Polymore/more-routing/blob/master/polymer-expressions.html#L25-33
I wonder if we could just put the extraDeps property on function itself? Since in JS that's no big deal. That way the filter would be callable.