ANCHOR SCROLL: 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
@petebacondarwin 
Right now we only need the "raw" DOM element (not the jqLite element), but still expect a jqLite element.
Should we modify the implementation so both raw and jqLite work (or do you think it complicates the "contract" for no real benefit) ?