Stop sticky from going outside constraint element.

This is so the sticky can be `position: fixed` up until it starts
to flow outside of a constraint element, at which point its
position is measured and it gets stuck at that point in the
container. It means a sidebar can have a sticky element so far as
the element is within the sidebar, which may not span the entire
height of the document.
v1.2.0
Set outOfBounds CSS in object literal instead of after.
Merge pull request #1 from incuna/stop-fixed-when-outside-constraint-height

Stop sticky from going outside constraint element.