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
Please, don't scream about this. I know :)
(It's ugly, it's not-Angular, it's hacky, it's hirting your eyes and I will remove it asap - just wanted to get some feedback on the offset approach.)