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 think toDOM/toModel is for two-way bindings. Conceivably they could have different "extra deps"?