adding Imager.add method

Ability to add elements after initialization.

```
var imager = new Imager(options);
imager.add(anotherDomElement);
imager.add('.element');
```

Possible use case: loading slider images on demand.

```
var imager = new Imager(options);
var slider = new SomeSlider(options);
slider.on('slide', function(element) { imager.add(element); });
```