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
Or for POC you could have just done:

```js
.config(['$anchorScrollProvider', function($anchorScrollProvider) {
  $anchorScrollProvider.setScrollOffset(function () { return 100; });
});
```

:-)