Trigger custom event slimmage-ready when slimmage has finished it's work

Trigger a custom event to indicate when slimmage is finished with replacing `<noscript>` to `<img>`. Handy if you rely on `<img>` to be present in the DOM before manipulating it with javascript. 

basic usage: 
```javascript
document.addEventListener("slimmage-ready", function () {  }, false);
```