second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
You should not need to manually register this URL. When you register the exporter in `varify/export/__init__.py` this endpoint should automatically be connected to the VcfExporter via [this resource](https://github.com/cbmi/serrano/blob/830889abeb38efd131cd1c26312d73c0e49eabc8/serrano/resources/exporter.py).