Facilitate async loading of quicklink.umd.js

Currently the readme suggests to include the script into the page via:

```html

quicklink();

```

or at `load` event via:

```html

window.addEventListener('load', () =>{
quicklink();
});

```

However, shouldn't the `quicklink.umd.js` script be loaded with `async` to begin with? The blocking script is not good for performance.

Ideally you should be able to do something like this, similar to the [AMP shadow doc API](https://github.com/ampproject/amphtml/blob/master/spec/amp-shadow-doc.md#using-shadow-doc-api):

```html

(window.quicklink = window.quicklink || []).push(function(quicklink) {
quicklink();
});
// Or...
window.addEventListener('load', () =>{
(window.quicklink = window.quicklink || []).push(function(quicklink) {
quicklink();
});
});

```