@Filter could be simpler

Stock Angular 1.3:

```
angular.module(...).filter("name", function(){
  return function(string, expression){ ... }
})
```

Since every filter follows this pattern, and stateful or injected filters are discouraged, perhaps A1 could do:

```
@Filter
export default function(string, expression){ ... }
```

(at the moment, the wrapping provider function is required)
