Replace ID in output VCF with --id option

[stephenturner]

Whether processing multiple files as fixed in #9, or when processing a single VCF with --output-vcf-path, the ID in the VCF file always takes the filename of the GTC file. Later we can change this with bcftools reheader or hack it with sed -i, but this adds additional complexity to our pipeline. It would be nice if there's an option when converting a single file to allow for an option like --id <newid> that would allow the ID in the VCF file to be set on the command line. Thanks Ryan.

cc @vpnagraj @punkrockscience