so, $parse('some angular expression') will return a getter function, which you call with `getterFn(scope, optionalLocals)`, and that will return the value of the expression (which might be a string, might be an object, hard to say).

Typically, the expression is also $watched, so that we can react to updates.

So, for example, 

```js
scope.$watch(attr.allowSelectors, function(newvalue) {
  // if it's a string, split on spaces or commas or something ...
  // then if you have an array, set the allowed selectors to the array.
});
```

Then just work on the cached array (if any, otherwise assume all are allowed or something).

I think that's a sensible approach, but the difficult thing is IIRC the Droppable class doesn't know about the directive Attributes object. Maybe it should be passed some extra info or something to make this possible.