Use `MutationObserver` for simpler re-parsing
Use `MutationObserver` for simpler re-parsing

When the list changes, the DOM changes. Hence, if the DOM isn't
changing, we have no need to re-parse the page and re-render the total.

This should be extremely responsive, less resource-intensive than
polling, and is much simpler to understand and maintain, to boot.

jasontbradshaw/amazon-wish-list-total,jasontbradshaw/amazon-wish-list-total