WIP - feat($anchorScroll): add support for configurable scroll offset

Add support for a configurable scroll offset to $anchorScrollProvider.
The offset is expressed in pixels and can be specified either as a fixed value or as a function that returns the offset it pixels dynamically.
(This is a POC and a WIP: no docs, no tests)

Related to #9368

Closes #2070, #9360
I think this should be:

```js
function scrollTo(elem) {
  if (elem) {
    elem.scrollIntoView();
    var offset = scrollOffsetGetter();
    if (offset) {
      $window.scrollBy(0, -1 * offset);
    }
  } else {
    $window.scrollTo(0, 0);
  }
}
```

Otherwise scrolling to the top never gets to 0, no?