[API design] Per-element options

[njmcode]

Want to be able to specify things like trigger point and other options on a per-element basis.

Potential options:

• trigger point ('top' [default], 'center', 'bottom', percentile, or pixel value)
• class to add/remove
• multiple trigger point logic e.g. 'add class X when top edge passed, remove it when bottom edge passed'
• single-use triggers vs re-triggerable
• callbacks/events to fire when trigger points are hit

Implementation ideas:

• data- params on DOM elements
• Additional param to addElement passing in extra options
• Additional methods to manipulate element options after registration

Ideas wanted!