second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
This line is very difficult to read, this can be broken up like this and it will be more readable IMO:

```python
selected_results = all_results.prefetch_related('sample', 'variant')\
    .prefetch_related('variant__chr')\
    .filter(label_criteria)\
    .filter(range_criteria)\
    .order_by('variant__chr__order', 'variant__pos')
```

Also, the `label_criteria` and `range_criteria` can be combined into a single `Q` object so that we only need a single filter call here instead of 2 calls for each `label_criteria` and `range_criteria`.