ImgLzy: jQuery selector caching

Cache jQuery selector when looping through all images in the article.

```
phantomas http://muppet.macbre.wikia-dev.com/wiki/Elmo?noexternals=1 --modules domQueries
```

Before:

```
 * class ".scroller": 11 queries
```

After:

```
 * class ".scroller": 2 queries
```

@prein / @michalroszka / @lizlux