Added `onAfterImageUploaded`

Hi,
I have an external module that handles the image uploads. The only way I could make it work with the easyMDE editor was to extend the prototype like this

```
EasyMDE.prototype.onAfterImageUploaded = function(imageUrl){
afterImageUploaded(this,imageUrl);
}
```

Then later in my code I would invoke it like this
```
let __easyMDE = new EasyMDE({...});
__easyMDE.onAfterImageUploaded(imageUrl);
```

I am wondering if this snippet could be added to the actual code base as I am sure many devs would love to have it.

Thanks,
Art