Allow for responsive handling of in-view-options by allowing expressions

[doelgonzo]

For a responsive site, we have a sticky header bar that changes height at smaller resolutions. I've been able to make angular-inview work for the majority of the usecase scenarios, but when the height changes, I need to adjust the viewportOffset to a different value at a smaller breakpoint. So, the question is why in-view-options doesn't allow an expression? I could then detect the responsive threshold change and change this offset value.

Thoughts or alternative suggestion?