chore(utils): modify positioning

* opts.margin and opts.offset are no longer recognized by the offset
  functions directly
  * margin will be baked into the Shadow DOM CSS
  * offset will be replaced with dX or dY (depending on the positioning function)
* opts.dX added to allow fine-tuning of the calculated X coordinate
* opts.dY added to allow fine-tuning of the calculated Y coordinate
* getPosition(offsetElement, refElement, config) -> getPosition(PositionConfig)
* getPositionWithArrow(offsetElement, refElement, config) -> getPositionWithArrow(PositionConfig)
* updated all positioned elements to consume `getPosition()` and
  `getPositionWithArrow()` with updated signature
* updated all positioned elements to be `position: fixed;`
  * still need to fix the logic to reposition on scroll

BREAKING: all functions return { x, y } object instead of [ x, y ] array
BREAKING: getPosition() and getPositionWithArrow() only accept a single PositionConfig argument

Author: Ryan A. Johnson