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
Yeah, that would jive better w/ filters within elements too. I was trying to follow along with [`toDOM`/`toModel`](https://github.com/Polymer/polymer-expressions/blob/master/tests/tests.js#L124-129) - not sure whether it's worth it though?