Adds option specifying a different element to use as 'bottom', past which the header is hidden.

[adrianschmidt]

This is probably a rather unusual use case, but we had the situation where we needed the header of one table to be shown while scrolling past both its own table and the next table below.

Thus, I added to option to specify which element should be used when calculating whether the "bottom" has been scrolled past or not.