Support `scope:` option to limit resumables

[seanpdoyle]

By default, calls to persistResumableFields() will query the document for elements with the (default or provided) selector:.

This commit adds support for specifying a scope: option to declare a ParentNode instance other than the document.

persistResumableFields(getPageID(), {
  scope: document.getElementById("my-scope"),
  selector: ".descendants-of-my-scope",
})