media_count parameter doesn't work on PostLooter.download

## Library version

```
instalooter 2.3.4
```

## Environment

* *Ubuntu 18.04*
* *Python 3.6.6*

## Error description - runtime

I would like to download only a single image from a post with multiple images. I used the `media_count` parameter from the corresponding method but it doesn't work. I still get all images from the given post.

### Reproducible test case

```python
from instalooter.looters import PostLooter
looter = PostLooter(
'BwKSVVVArhP',
get_videos=True,
jobs=1,
template="{code}_{id}",
)
looter.download(media_count=1, destination='.')
```

### Expected behaviour

*Download a single image from given post*

### Actual behaviour

*Download all images from given post*