Enable downloading async content per site

[bunglegrind]

A boolean option in the settings should be enough.

The following code must be injected in the page:

const originalIntersectionObserver = window.IntersectionObserver;
window.IntersectionObserver = function (handle, opt) {
        return new originalIntersectionObserver(function(...args) {
             setTimeout(handle, 1000, ...args);
        }, {...opt, root: document.body});
}