second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
You can import QueueTestCase from [this test](https://github.com/cbmi/varify/blob/52323813b9ecbfe21959fa68866fcdd758fe9456/tests/cases/sample_load_process/tests.py#L21) to avoid recreating the same code here. Ideally this should be moved out into the base case but for now you can just import it from there until I move it over