Provide more control over scrollSensitivity option of interactions

[tilwinjoy]

It'd be great to have an option for specifying the scroll offset for each edge.

Right now the scrollSensitivity option only accepts a number. It'd be awesomeIf it accepts an object of the form

{
 top:50,
 right:50,
 bottom:100,
 left:0
}

or an array containing offsets corresponding to each edge like [50,50,100,0].

This is especially useful when there are fixed/sticky positioned elements like headers and sidebars overlapping interactive items. The scroll should start as soon as the mouse pointer reaches the sticky item.

Currently the scrollSensitivity option only provides an option to set the same offset for all edges, which is bad when we need to specify offset only for one edge, for example a sticky header. Other edges should behave normally.